Aesth Avat atalino eae hangs eaoge a i] +7 Worl Rarny vty Bie ‘ renin } ott VAlUNisencn inv ayieas ys rt in Pa ays Schr parts ty Tam Lewy ah wi i PEL te ener hry Wiese! dst, Mash h Si IAA Snanen re fy se , ri iy pry ati ys 0 ine ‘4 ue tG ” wih ag ‘y Dab TT Avge, IOOabe es. ¢ Dorarine’ “p Ta phsale Maile yay Ay a pu yataye PL PItON We bye nha ¢ Deon rane aes i NGO Ar heaen dmeder Ve AGNES ye , ed Fk Vike Mea : AAEM np Sita UE IA: viii Aga sin tks kehanes ietcaghin ty: sgt, CREP bann Yeates Nini) Medd dhe Wares tae Ling SOeCe rin eanier pt ey EY Shara s ATRIA VEN KO Bwana rs UAT 3 1761 O2484700 & » M roe eet d Parte Piety ther PSM Soke ish evap oenyy 4 PaRSS crs a RVerg Seed vite Sarl fi *seAlalige ft ; Fehesee RL . an CEUs eereag dnote Teer an yy Breet PErDe Leer Para rien 97 OME YEE) Pease al tcdaNEs mee edit Chee 2 eriwnntet re Sous PPM TAA Nn mids ST meng ed, 3 i pierre CMEC INT Cate t Tay ftveg PE WRti geheag Bre teeny CUR ayo © dE ob ago Chon Metanes Ayn edns, see nv ty é Bers nh TPP Oa pepetteaeay ad oe rr (eee VE e FeAl ests aera natty : AONE ENER tla chin ony eats seve le hte i ae oe Ri fel er nels Vato SRD. 5 .: jie hid omene eae e. teats tQea, = Pa HSINGB see Be Ame enve Rin yoy Mi Na clehimectene as ee 2 ‘ WANE Ug ag Tr twne Vie hee he Pra letsll su tend USE Meecha tg USER VIER “ Pde sh EP ALUAG ST EU Site Ay ered ey toy te verds ates oleae! Gh Nadas ake if . age ht 5 METALS Ar Mess Aaiy Mew a Os EVISTA LG 4 A (Aer Sivas Siesevas Se bets Lend i pitas MINS Parsee] ESPEN veg Re rete PIN th dA ORE hs poh tity © EV 4 tert ne erat eng oath ‘ The brat Ps tales kek cipelteny We fies en tans i serethes ee acke t tte sa ieok the Pent aad A Pha ive Unby uSir Cen | frond exit stag a iby bales Mass its Beets puekie M14 a rae) spat esckras epg tui or eve soatteng LU eA ery ers hoy ien ry HVAC N DPE tall tgs 6 coe x Ea, Pane St ran OB La Ee © ri a ate. aayee i ud MG APS IAW Ce oe tok ra aed BERS RODEI RY Tt fi * Settle Pints Dott VEEP AP Akos tap oo Avis cd sean Sepatene tas Jeune Uris. sha Ye Sdenwhinad T Hate Copan ache phat pea Ce ea ra © 4 BSaNE ete ec geet ‘ . vile SAeLa enV NPR tale Cy igh re IN) Foe eehetty in | AINE LS ROR Le RECA TN he DG tins ha Pele atin Vor bea ye ig et Piers 9 21 AY yt as Let ape) vesraral lean, f ue Aca Talat ts Tas C94 eh Shy oy eee} < We es TD es pigt ele viaty la eA Road Wes HbA Pee GS ata ted le DOA WN ee ns oe ce Ad hid abt ates Be Hs Sa PLAC Pee, ne ae hed +4 si Paris Beit) “nw ae dee MAL Bias wae Tieton (ey y f eT ee od) : rien as Digitized by the Internet Archive in 2007 with funding from Microsoft Corporation http://www.archive.org/details/effectsofcrossse00darwuoft Pe ae PE RO SOF CROSS AND SELF FERTILISATION IN THE VEGETABLE KINGDOM BY CHARLES DARWIN, LL.D): F.R.S. ! ie qt oa, | ae LONDON JOHN MURRAY, ALBEMARLE STREET, W. 1916 First EDITION . . Reprinted ° : . r Reprinted . . : . Reprinted . : : ° Reprinted. ‘ . ‘ ° SECOND EDITION . . ° . Ad rights reserved . Deceméber, June, December, June, » September, August, 1876 1878 1887 1891 1900 1g16 CONTENTS. ne CHAP TE Rl: InTropucTory REMARKS. Various means which favour or determine the cross-fertilisation 6 plants — Benefits derived from cross-fertilisation — Self-fertilisa- tion favourable to the propagation of the species — Brief history of the subject — Object of the experiments, and the manner in which they were tried—Statistical value of the measurements— The experiments carried on during several successive genera- tions —*Nature of the relationship of the plants in the later generations — Uniformity of the conditions to which the plants were subjected — Some apparent and some real causes of error — Amount of pollen employed— Arrangement of the work — Importance of the conclusions .. .. .. .. Page 1-27 CHAPTER II. CoNVOLVULACE. Ipomeea purpurea, comparison of the height and fertility of the crossed and self-fertilised plants during ten successive genera- tions — Greater constitutional vigour of the crossed plants— The effects on the offspring of crossing different flowers on the same plant, instead of crossing distinct individuals — The effects of a cross with a fresh stock—The descendants of the self-fertilised plant named Hero—Summary on the growth, vigour, and fertility of the successive crossed and self-fertilised generations — Small amount of pollen in the anthers of the self- iv CONTENTS. fertilised plants of the later generations, and the sterility of their first-produced flowers— Uniform colour of the flowers produced by the self-fertilised plants —-The advantage from a cross between two distinct plants depends on their differing in COUSELDUTION Yep si Vege ire! hpemy tae ote.) wali) oe oe BR Re See CHA PDE R: £11: ScROPHULARIACEH, GESNERIACEH, LABIATH, ETO. Mimulus luteus; height, vigour, and fertility of the crossed and self-fertilised plants of the first four generations — Appearance of a new, tall, and highly self-fertile variety — Offspring from a cross between self-fertilised plants — Effects of a cross.with a fresh stock — Effects of crossing flowers on the same plant — Summary on Mimulus luteus — Digitalis purpurea, superiority of the crossed plants — Effects of crossing flowers on the same plant — Calceolaria — Linaria vulgaris — Verbascum thapsus — Vandellia nummularifolia — Cleistogamic flowers— Gesneria pen- dulina — Salvia coccinea — Origanum vulgare, great increase of the crossed plants by stolons — Thunbergia alata -- 63-97 CHAPTER EY. CRUCIFERA, PAPAVERACEH, RESEDACER, ETC. Brassica oleracea, crossed and self-fertilised plants — Great effect of a cross with a fresh stock on the weight of the offspring — Iberis umbellata— Papaver vagum— Eschscholtzia californica, seed- lings from a cross with a fresh stock not more vigorous, but more fertile than the self-fertilised seedlings — Reseda lutea and odorata, many individuals sterile with their own pollen — Viola tricolor, wonderful effects of a cross— Adonis exstivalis— Delphinium consolida — Viscaria oculata, crossed plants hardly taller, but more fertile than the self-fertilised — Dianthus caryophyllus, crossed and self-fertilised plants compared for four generations — Great effects of a cross with a fresh stock — Uniform colour of the flowers on the self-fertilised plants — Eltbiscus afrioamms: 11) G56 G06)! icc) MP ae) Wl eanasen | Geeabae CONTENTS. Vv CHAPTER V. GERANIACE, LEGUMINOSH, ONAGRACE#, BC. Pelargonium zonale, a cross between plants propagated by cuttings does no good —Tropeolum minus—Limnanthes douglasii— Lupinus luteus and pilosus— Phaseolus multiflorus and vul- garis—Lathyrus odoratus, varieties of, never naturally inter- cross in England — Pisum sativum, varieties of, rarely inter- cross, but a cross between them highly beneficial—Sarothamnus scoparius, wonderful effects of a cross—Ononis minutissima, cleistogamic flowers of — Summary onthe Leguminosz — Clarkia elegans — Bartonia aurea — Passiflora gracilis — Apium petrose- linum —Scabiosa atropurpurea — Lactuca sativa — Specularia speculum — Lobelia ramosa, advantages of a cross during two cenerations — Lobelia fulgens — Nemophila insignis, great ad- vantages of a cross — Borago officinalis — Nolana prostrata. Page 142-187 CHAPTER VI. SoLANACES, PRIMULACEH, PoLYGONEA, ETC. Petunia violacea, crossed and self-fertilised plants compared for four generations — Effects of a cross with a fresh stock — Uniform colour of the flowers on the self-fertilised plants of the fourth generation — Nicotiana tabacum, crossed and _ self-fertilised plants of equal height — Great effects of a cross with a distinct ' sub-variety on the height, but not on the fertility, of the off- spring — Cyclamen persicum, crossed seedlings greatly superior to the self-fertilisea — Anagallis collina — Primula veris — Equal-styled variety of Primula veris, fertility of, greatly increased by a cross with a fresh stock—Fagopyrum esculentum — Beta vulgaris — Canna warscewiczi, crossed and self-fertilised plants of equal height — Zea mays — Phalaris canariensis. 188-237 v1 CONTENTS. CHAPTER VII. SuMMARY OF THE HEIGHTS AND WEIGHTS or THE CROSSED AND SELF-FERTILISED PLANTS. Number of species and plants measured— Tables given — Pre~ liminary remarks on the offspring of plants crossed by a fresh stock — Thirteen cases specially considered — The effects of crossing a self-fertilised plant either by another self-fertilised plant or by an intercrossed plant of the old stock — Summary of the results— Preliminary remarks on the crossed and self- fertilised plants of the same stock — The twenty-six exceptionas cases considered, in which the crossed plants did not exceed greatly in height the self-fertilised — Most of these cases shown not to be real exceptions to the rule that cross-fertilisation is beneficial— Summary of results— Relative weights of the crossed and self-fertilised plants .. .. .. .. Page 238-284 CHAPTER VIII, DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS. Greater constitutional vigour of crossed plants— The effects of great crowding — Competition with other kinds of plants — Self-fer- tilised plants more liable to premature death — Crossed plants generally flower before the self-fertilised — Negative effects of intercrossing flowers on the same plant— Cases described — Transmission of the good effects of a cross to later generations — Effects of crossing plants of closely related parentage — Uniform colour of the flowers on plants self-fertilised during several generations and cultivated under similar conditions. 285-311 CHAPTER IX. THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE PRODUCTION OF SEEDS. Fertility of plants of crossed and self-fertilised parentage, both lots being fertilised in the same manner— Fertility of the parent- plants when first crossed and self-fertilised, and of their crossed CONTENTS. vu and self-furtilised offspring when again crossed and self--fertilised — Comparison of the fertility of flowers fertilised with their own pollen and with that from other flowers on the same plant — Self-sterile plants — Causes of self-sterility — The appearance of highly self-fertile varieties — Self-fertilisation apparently in some respects beneficial, independently of the assured produc- tion of seeds— Relative weights and rates of germination of seeds from crossed and self-fertilised flowers .. Page 312-355 CHEAP ER axe Means or FERTILISATION. Sterility and fertility of plants when insects are excluded — The means by which flowers are cross-fertilised — Structures favour- able to self-fertilisation — Relation between the structure and conspicuousness of flowers, the visits of insects, and the advan- tages of cross-fertilisation —The means by which flowers are fertilised with pollen from a distinct plant — Greater fertilising power of such pollen — Anemophilous species —Conversion of anemophilous species into entomophilous — Origin of nectar — Anemophilous plants generally have their sexes separated — Conversion of diclinous into hermaphrodite flowers — Trees often have their sexes separated... .. .. .. « 856-417 CHAPTER XI. Tue Hasits oF INSECTS IN RELATION TO THE FERTILISATION or FLOWERs. Insects visit the flowers of the same species as long as they can — Cause of this habit—Means by which bees recognise the flowers of the same species—Sudden secretion of nectar — Nectar of certain flowers unattractive to certain insects —In- dustry of bees, and the number of flowers visited within a short time — Perforation of the corolla by bees — Skill shown in the operation — Hive-bees profit by the holes made by humble-bees — Effects of habit —~ The motive for perforating flowers to save time — Flowers growing in crowded masses chiefly perforated. 418-438 vil CONTENTS. CHAPTER AII. GENERAL RESULTS. Cross-fertilisation proved to be beneficial, and self-fertilisation in- jurious — Allied species differ greatly in the means by which cross-fertilisation is favoured and self-fertilisation avoided — The benefits and evils of the two processes depend on the degree of differentiation in the sexual elements —'Lhe evil effects not due to.the combination of morbid tendencies in the parents — Nature of the conditions to which plants are subjected when growing near together in a state of nature or under culture, and the effects of such conditions — Theoretical considerations with respect to the interaction of differentiated sexual elements — Practical lessons — Genesis of the two sexes — Close correspon- dence between the effects of cross-fertilisation and self-fertilisa- ‘tion, and of the legitimate and illegitimate unions of hetero- styled plants, in comparison with hybrid unions Page 439-474 EMBER! ica! es a6) ae 7 ans see, Paar oe ee ese tone THE EFFECTS OF CROSS AND SELF-FERTILISATION. CHAPTER I. INTRODUCTORY REMARKS. Various means which favour or determine the cross-fertilisation of plants—Benefits derived from cross-fertilisation—Self-fertilisation favourable to the propagation of the species—Brief history of the subject—Object of the experiments, and the manner in which they were tried—Statistical value of the measurements—The experi- ments carried on during several successive generations—Nature of the relationship of the plants in the later generations—Unifor- mity of the conditions to which the plants were subjected—Some apparent and some real causes of error—Amount of pollen em- ployed—Arrangement of the work—Importance of the conclusions. THERE is weighty and abundant evidence that the flowers of most kinds of plants are constructed so as to be occasionally or habitually cross-fertilised by pollen from another flower, produced either by the same plant, or generally, as we shall hereafter see reason to believe, by a distinct plant. Cross-fertilisa- tion is sometimes ensured by the sexes being separated, and in a large number of cases by the pollen and stigma of the same flower being ‘matured at different times. Such plants are called dichogamous, and have been divided into two sub-classes: proterandrous species, B = 2 INTRODUCTORY REMARKS. Cuar. I. in which the pollen is mature before the stigma, and proterogynous species, in which the reverse occurs; this latter form of dichogamy not being nearly so common as the other. Cross-fertilisation is also en- sured, in many cases, by mechanical contrivances of wonderful beauty, preventing the impregnation of the flowers by their own pollen. There is a small class of plants, which I have called dimorphic and trimorphie, but to which Hildebrand has given the more appro- priate name of heterostyled; this class consists of plants presenting two or three distinct forms, adapted for reciprocal fertilisation, so that, like plants with separate sexes, they can hardly fail to be intercrossed in each generation. The male and female organs of some flowers are irritable, and the insects which touch them get dusted with pollen, which is thus transported. to other flowers. Again, there is a class, in which the ovules absolutely refuse to be fertilised by pollen from the same plant, but can be fertilised by pollen from any other individual of the same species. There are also very many species which are partially sterile with their own pollen. Lastly, there is a large class in which the flowers present no apparent obstacle of any kind to self-fertilisation, nevertheless these plants are frequently intercrossed, owing to the prepotency of pollen from another individual or variety over the plant’s own pollen. As plants are adapted by such diversified and effec- tive means for cross-fertilisation, it might have been inferred from this fact alone that they derived some great advantage from the process ; and it is the object of the present work to show the nature and importance of the benefits thus derived. There are, however, some exceptions to the rule of plants being constructed so as to allow of or to favour cross-fertilisation, for some Cuap. I. INTRODUCTORY REMARKS 3 few plants seem to be invariably self-fertilised; yet even these retain traces of having been formerly adapted for cross-fertilisation. ‘These exceptions need not make us doubt the truth of the above rule, any more than the existence of some few plants which pro- duce flowers, and yet never set seed, should make us doubt that flowers are adapted for the production of seed and the propagation of the species. We should always keep in mind the obvious fact that the production of seed is the chief end of the act of fertilisation; and that this end can be gained by hermaphrodite plants with incomparably greater certainty by self-fertilisation, than by the union of the sexual elements belonging to two distinct flowers or plants. Yet it is as unmistakably plain that innu- merable flowers are adapted for cross-fertilisation, as that the teeth and talons of a carnivorous animal are adapted for catching prey ; or that the plumes, wings, and hooks of a seed are adapted for its dissemination. Flowers, therefore, are constructed so as to gain two objects which are, to a certain extent, antagonistic, and this explains many apparent anomalies in their struc- ture. The close proximity of the anthers to the stigma in a multitude of species favours, and often leads, to self-fertilisation ; but this end could have been gained far more safely if the flowers had been completely closed, for then the pollen would not have been injured by the rain or devoured by insects, as often happens. Moreover, in this case, a very small quantity of pollen would have been sufficient for fertilisation, instead of millions of grains being produced. But the openness - of the flower and the production of a great and ap- parently wasteful amount of pollen are necessary for - eross-fertilisation. These remarks are well illustrated by the plants called cleistogamic, which bear on the B 2 4 INTRODUCTORY REMARKS. Cuar. I. same stock two kinds of flowers. The flowers of the one kind are minute and completely closed, so that they cannot possibly be crossed; but they are abundantly fertile, although producing an extremely small quantity of pollen. The flowers of the other kind produce much pollen and are open; and these can be, and often are, cross-fertilised. Hermann Miller has also made the remarkable discovery that there are some plants which exist under two forms; that 1s, produce on distinct stocks two kinds of hermaphrodite flowers. The one form bears small flowers constructed for self-fertilisation ; whilst the other bears larger and much more conspicuous flowers plainly constructed for cross-fertilisation by the aid of insects; and without their aid these produce no seed. The adaptation of flowers for cross-fertilisation is a subject which has interested me for the last thirty- seven years, and I have collected a large mass of ob- servations, but these are now rendered superfluous by the many excellent works which have been lately pub- lished. In the year 1857 I wrote* a short paper on the fertilisation of the kidney bean; and in 1862 my work ‘On the Contrivances by which British and Foreign Orchids are Fertilised by Insects’ appeared. It seemed to me a better plan to work out one group of plants as carefully as I could, rather than to pub- lish many miscellaneous and imperfect observations. My present work is the complement of that on Orchids, in which it was shown how admirably these plants are constructed so as to permit of, or to favour, or to necessitate cross-fertilisation. The adaptations * «Gardeners’ Chronicle” 1857, ‘Annals and Mag. of Nat. Hist, p. 725, and 1858, p. 828. Also 3rd series, vol. ii. 1858, p. 462. Cup. I, INTRODUCTORY REMARKS. a for cross-fertilisation are perhaps more obvious in the Orchidez than in any other group of plants, but it is an error to speak of them, as some authors have done, as an exceptional.case. The lever-like action of the stamens of Salvia (described by Hildebrand, Dr. W. Ogle, and others), by which the anthers are depressed and rubbed on the backs of bees, shows as perfect a structure as can be found in any orchid. Papilion- aceous flowers, as described by various authors—for instance, by Mr. T. H. Farrer—offer innumerable curious adaptations for cross-fertilisation. The case of Posoqueria fragrans (one of the Rubiacez), is as won- derful as that of* the most wonderful orchid. The stamens, according to Fritz Miiller,* are irritable, so that as soon as a moth visits a flower, the anthers ex- plode and cover the insect with pollen; one of the filaments which is broader than the others then moves and closes the flower for about twelve hours, after which time it resumes its original position. Thus the stigma cannot be fertilised by pollen from the same flower, but only by that brought by a moth from some other flower. Endless other beautiful contrivances for this same purpose could be specified. Long before I had attended to the fertilisation of flowers, a remarkable book appeared in 1793 in Ger- . many, ‘Das Entdeckte Geheimniss der Natur,’ by C. K. Sprengel, in which he clearly proved by innumer- able observations, how essential a part insects play in the fertilisation of many plants. But he was in ad- vance of his age, and his discoveries were for a long time neglected. Since the appearance of my book on Orchids, many excellent works on the fertilisation of flowers, such as those by Hildebrand, Delpino, Axell, * ‘Botanische Zeitung,’ 1866, p, 129, 6 INTRODUCTORY REMARKS. Cuar. I and Hermann Miiller,* and numerous shorter papers, have been published.. A list would occupy several pages, and this is not the proper place to give their titles, as we are not here concerned with the means, but with the results of cross-fertilisation. No one ° who feels interest in the mechanism by which nature effects her ends, can read these books and memoirs without the most lively interest. From my own observations on plants, guided to a certain extent by the experience of the breeders of animals, I became convinced many years ago that it isa general law of nature that flowers are adapted to be crossed, at least occasionally, by pollen from a distinct plant. Sprengel at times foresaw this law, but only partially, for it does not appear that he was aware that there was any difference in power between pollen from the same plant and from a distinct plant. In the introduction to his book (p. 4) he says, as the sexes are separated in so many flowers, and as so many other flowers are dichogzamous, “it appears that nature has. oD ’ not willed that any one flower should be fertilised by its own pollen.” Nevertheless, he was far from keeping this conclusion always before his mind, or he did not * Sir John Lubbock has given an interesting summary of the whole subject in his ‘ British Wild Tlowers considered in relation to Insects,’ 1875. Hermann Miiller’s work ‘Die Befruchtung der Blu- men durch Insekten,’ 1873, con- tains an immense number of original observations and gene- ralisations. It is, moreover, in- valuable as a repertory with re- ferences to almost everything which has been published on the subject. His work differs from that of all others in specifying what kinds of insects, as far as known, visit the flowers of each species. He likewise enters on new ground, by showing not only that flowers are adapted for their own good to the visits of certain insects; but that the insects themselves are excellently adapted for procuring nectar or pollen from certain flowers. The value of H. Miiller’s work can hardly be | over-estimated, and it is much to be desired that it should be trans- lated into English. Severin Axell’s work is written in Swedish, so that I have not been able to read it, a a eee Ouar. I. INTRODUCTORY REMARKS. 7 see its full importance, as may be perceived by any- one who will read his observations carefully; and he consequently mistook the meaning of various struc- tures. But his discoveries are so numerous and his work so excellent, that he can well afford to bear a small amount of blame. A most capable judge, H. Miller, likewise says :* “tis remarkable in how very many cases Sprengel rightly perceived that pollen is necessarily transported to the stigmas of other flowers of the same species by the insects which visit them, and yet did not imagine that this transportation was of any service to the plants themselves.” Andrew Knight saw the truth much more clearly, for he remarks,t “ Nature intended that a sexual in- tercourse should take place between neighbouring plants of the same species.’ After alluding to the various means by which pollen is transported from flower to flower, as far as was then imperfectly known, he adds, “ Nature has something more in view than that its own proper males should fecundate each blossom.” In 1811 Kélreuter plainly hinted at the same law, as did afterwards another famous hybridiser of plants, Herbert.t But none of these distinguished observers appear to have been sufficiently impressed with the * «Die Befruchtung der Blu- men,’ 1873, p.4. His words are: “Es ist merkwiirdig, in wie zahl- reichen Fallen Sprengel richtig erkannte, dass durch die Besuch- enden Insekten der Bliithenstaub mit Nothwendigkeit auf die Nar- ben anderer Bliithen derselben Art iibertragen wird, ohne auf die Vermuthung zu kommen, dass in dieser Wirkung der Nutzen des Insektenbesuches fiir die Pflanzen selbst gesucht werden miisse.” + ‘Philosophical Transactions,’ 1799, p.. 202, t¢ Kolreuter, ‘Meém. de l’Acad. de St. Pétersbourg,’ tom. iii. 1809 (published 1811), p. 197. After showing how well the Malvacess are adapted for cross-fertilisation, he asks, ‘‘ An id aliquid in recessu habeat, quod hujuseemodi flores nunquam proprio suo pulvere, sed semper eo aliarum sus speciei impregnentur, merito queritur? Certe natura nil facit frustra.” Herbert, ‘Amaryllidaces, with a Treatise on Cross-bred Vege tables,’ 1837, ie INTRODUCTORY REMARKS. | Cuar. I. truth and generality of the law, so as to insist on it and impress their belief on others. In 1862 I summed up my observations on Orchids by saying that nature “abhors perpetual self-fertilisa- tion.” If the word perpetual had been omitted, the aphorism would have been false. As it stands, I believe that it is true, though perhaps rather too strongly expressed; and I should have added the self-evident proposition that the propagation of the species, whether by self-fertilisation or by cross-fertili- sation, or asexually by buds, stolons, &c. is of paramount importance. Hermann Miller has done excellent service by insisting repeatedly on this latter point. It often occurred to me that it would be advisable to try whether seedlings from cross-fertilised flowers were in any way superior to those from self-fertilised flowers. But as no instance was known with animals of any evil appearing in a single generation from the closest possible interbreeding, that is between brothers and sisters, I thought that the same rule would hold good with plants; and that it would be necessary at the sacrifice of too much time to self-fertilise and inter- cross plants during several successive generations, in order to arrive at any result. I ought to have re- flected that such elaborate provisions favouring cross- fertilisation, as we see in innumerable plants, would not have been acquired for the sake of gaining a distant and slight advantage, or of avoiding a distant and slight evil. Moreover, the fertilisation of a flower by its own pollen corresponds to a closer form of inter- breeding than is possible with ordinary bi-sexual animals; so that an earlier result might have been expected. I was at last led to make the experiments recorded in the present volume from the following circumstance, Cuap. I. INTRODUCTORY REMARKS. 9 For the sake of determining certain points with respect to inheritance, and without any thought of the effects of close interbreeding, I raised close together two large beds of self-fertilised and crossed seedlings from the same plant of Linaria vulgaris. To my surprise, the crossed plants when fully grown were plainly taller and more vigorous than the self-fertilised ones. Bees incessantly visit the flowers of this Linaria and carry pollen from one to the other; and if insects are ex- cluded, the flowers produce extremely few seeds; so that the wild plants from which my seedlings were raised must have been intercrossed during all previous generations. It seemed therefore quite incredible that the difference between the two beds of seedlings could have been due to a single act of self-fertilisation ; and I attributed the result to the self-fertilised seeds not having been well ripened, improbable as it was that all should have been in this state, or to some other accidental and inexplicable cause. During the next year, I raised for the same purpose as before two large beds close together of self-fertilised and crossed seed- lings from the carnation, Dianthus caryophyllus. This plant, like the Linaria, is almost sterile if insects are excluded; and we may draw the same inference as before, namely, that the parent-plants must have been intercrossed during every or almost every previous generation. Nevertheless, the self-fertilised seedlings were plainly inferior in height and vigour to the crossed. My attention was now thoroughly aroused, for I could hardly doubt that the difference between the two beds was due to the one set being the offspring of crossed, and the other of self-fertilised flowers. Accordingly I selected almost by hazard two other plants, which happened to be in flower in the greenhouse, namely, 10 INTRODUCTORY REMARKS. Cuar. L Mimulus luteus and Ipomea purpurea, both of which, unlike the Linaria and Dianthus, are highly self- fertile if insects are excluded. Some flowers on a single plant of both species were fertilised with their own pollen, and others were crossed with pollen from a distinct individual; both plants being protected by a net from insects. The crossed and self-fertilised seeds thus produced were sown on opposite sides of the same pots, and treated in all respects alike; and the plants when fully grown were measured and compared. With both species, as in the cases of the Linaria and Dianthus, the crossed seedlings were conspicuously superior in height and in other ways to the self- fertilised. I therefore determined to begin a long series of experiments with various plants, and these were continued for the following eleven years; and we shall see that in a large majority of cases the crossed beat the self-fertilised plants. Several of the excep- tional cases, moreover, in which the crossed plants were not victorious, can be explained. It should be observed that I have spoken for the sake of brevity, and shall continue to do so, of crossed and self-fertilised seeds, seedlings, or plants; these terms implying that they are the product of crossed or self-fertilised flowers. Cross-fertilisation always means a cross between distinct plants which were raised from seeds and not from cuttings or buds. Self-fertilisation always implies that the flowers in question were im- pregnated with their own pollen. My experiments were tried in the following manner. A single plant, if it produced a sufficiency of flowers, or two or three plants were placed under a net stretched on a frame, and large enough to cover the plant (together with the pot, when one was used) without touching it. This latter point is important, for if Cap. I. INTRODUCTORY REMARKS, 11 the flowers touch the net they may be cross-fertilised by bees, as I have known to happen; and when the net is wet the pollen may be injured. I used at first “ white cotton net,” with very fine meshes, but after- wards a kind of net with meshes one-tenth of an inch in diameter; and this I found by experience effectually excluded all insects excepting Thrips, which no net will exclude. On the plants thus protected several flowers were marked, and were fertilised with their own pollen; and an equal number on the same plants, marked in a different manner, were at the same time crossed with pollen from a distinct plant. The erossed flowers were never castrated, in order to make the experiments as like as possible to what occurs under nature with plants fertilised by the aid of insects. ‘Therefore, some of the flowers which were crossed may have failed to be thus fertilised, and afterwards have been self-fertilised. But this and some other sources of error will presently be discussed. In some few cases of spontaneously self-fertile species, the flowers were allowed to fertilise themselves under the net; and in still fewer cases uncovered plants were allowed to be freely crossed by the insects which in- cessantly visited them. ‘There are some great advan- tages and some disadvantages in my having occasion- ally varied my method of proceeding ; but when there was any difference in the treatment, it is always so stated under the head of each species. Care was taken that the seeds were thoroughly ripened before being gathered. Afterwards the crossed and self-fertilised seeds were in most cases placed on damp sand on opposite sides of a glass tumbler covered by a glass plate, with a partition between the two lots ; and the glass was placed on the chimney-piece in a warm room. I could thus observe the germination of 12 INTRODUCTORY REMARKS. Cuar. L the seeds. Sometimes a few would germinate on one side before any on the other, and these were thrown away. But as often as a pair germinated at the same time, they were planted on opposite sides of a pot, with a superficial partition between the two; and I thus proceeded until from half-a-dozen to a score or more seedlings of exactly the same age were planted on the opposite sides of several pots. If one of the young seedlings became sickly or was in any way injured, it was pulled up and thrown away, as well as its antagonist on the opposite side of the same pot. As a large number of seeds were placed on the sand to germinate, many remained after the pairs had been selected, some of which were in a state of germination and others not so; and these were sown crowded together on the opposite sides of one or two rather larger pots, or sometimes in two long rows out of doors. In these cases there was the most severe struggle for life among the crossed seedlings on one side of the pot, and the self-fertilised seedlings on the other side, and between the two lots which grew in competition in the same pot. A vast number soon perished, and the tallest of the survivors on both sides when fully grown were measured. Plants treated in this manner, were subjected to nearly the same conditions as those growing in a state of nature, which have to struggle to maturity in the midst of a host of competitors. On other occasions, from the want of time, the seeds, instead of being allowed to germinate on damp sand, were sown on the opposite sides of pots, and the fully grown plants measured. But this plan is less accurate, as the seeds sometimes germinated more quickly on one side than on the other. It was however necessary to act in this manner with some few species, as certain Cuar. I. INTRODUCTORY REMARKS. 13 kinds of seeds would not germinate well when exposed to the light; though the glasses containing them were kept on the chimney-piece on one side of a room, and some way from the two windows which faced the N.E.* . The soil in the pots in which the seedlings were planted, or the seeds sown, was well mixed, so as to be uniform in composition. The plants on the two sides were always watered at the same time and as equally as possible; and even if this had not been done, the water would have spread almost equally to both sides, as the pots were not large. The crossed and self-fertilised plants were separated by a super- ficial partition, which was always kept directed towards the chief source of the light, so that the plants on both sides were equally illuminated. I do not believe it possible that two sets of plants could have been sub- jected to more closely similar conditions, than were my crossed and self-fertilised seedlings, as grown in the above described manner. In comparing the two sets, the eye alone was never trusted. Generally the height of every plant on both sides was carefully measured, often more than once, viz., whilst young, sometimes again when older, and finally when fully or almost fully grown. But in some cases, which are always specified, owing to the want of time, only one or two of the tallest plants on each side were measured. This plan, which is not a good one, was never followed (except with the crowded * This occurred in the plainest manner with the seeds of Papaver vagum and Delphinium consolida, and less plainly with those of Adonis xstivalis and Ononis minu- tissima. Rarely more than one or two of the seeds of these four specics germinated on the bare sand, though left there for some weeks ; but when these same seeds were placed on earth in pots, and covered with a thin layer of sand, they germinated immediately in large numbers. 14 INTRODUCTORY REMARKS. Cnap, 4. plants raised from the seeds remaining after the pairs had been planted) unless the tallest plants on ach side seemed fairly to represent the average difference between those on both sides. It has, however, some great advantages, as sickly or accidentally injured plants, or the offspring of ill-ripened seeds, are thus eliminated. When the tallest plants alone on each side were measured, their average height of course exceeds that of all the plants on the same side taken togethex But in the case of the much crowded plants raisea from the remaining seeds, the average height of the tallest plants was less than that of the plants in pairs, owing to the unfavourable conditions to which they were subjected from being greatly crowded. For our purpose, however, of the comparison of the crossed and self-fertilised plants, their absolute height signifies little. As the plants were measured by an ordinary English standard divided into inches and eighths of an inch, I have not thought it worth while to change the frac- tions into decimals. The average or mean heights were calculated in the ordinary rough method by adding up the measurements of all, and dividing the product by the number of plants measured; the result being here given in inches and decimals. As the different species grow to various heights, I have always for the sake of easy comparison given in addition the average height of the crossed plants of each species taken as 100, and have calculated the average height of the self-fertilised plant in relation to this standard. With respect to the crowded plants raised from the seeds remaining after the pairs had been planted, and of which only some of the tallest on each side were measured, I have not thought it worth while to complicate the results by giving separate averages Cuar. I. INTRODUCTORY REMARKS. 15 for them and for the pairs, but have added up all their heights, and thus obtained a single average. I long doubted whether it was worth while to give the measurements of each separate plant, but have decided to do so, in order that it may be seen that the superiority of the crossed plants over the self-fertilised, does not commonly depend on the presence of two or three extra fine plants on the one side, or of a few very poor plants on the other side. Although several observers have insisted in general terms on the off- spring from intercrossed varieties being superior to either parent-form, no precise measurements have been given ;* and I have met with no observations on the effects of crossing and self-fertilising the indi- viduals of the same variety. Moreover, experiments ot this kind require so much time—mine haying been continued during eleven years—that they are not likely soon to be repeated. As only a moderate number of crossed and_self- fertilised plants were measured, it was of great impor- tance to me to learn how far the averages were trust- worthy. I therefore asked Mr. Galton, who has had much experience in statistical researches, to examine some of my tables of measurements, seven in number, namely, those of Ipomcea, Digitalis, Reseda lutea, Viola, Limnanthes, Petunia, and Zea. I may premise that if we took by chance a dozen or score of men belonging to two nations and measured them, it would I prestme be very rash to form any judgment from such small numbers on their average heights. But the case is somewhat different with my crossed and self-fertilised plants, as they were of exactly the same * A summary of these state- and Plants under Domestication, taents, with references, may be chap. ong, 2nd edit., 1875, vol. found in my ‘ Variation of Animals ii. p. 109. 16 INTRODUCTORY REMARKS. Cuar. L age, were subjected from first to last to the same conditions, and were descended from the same parents. When only from two to six pairs of plants were measured, the results are manifestly of little or no value, except in so far as they confirm and are con- firmed by experiments made on a larger scale with other species. I will now give the report on the seven tables of measurements, which Mr. Galton has had the great kindness to draw up for me. “T have examined the measurements of the plants with care, and by many statistical methods, to find out how far the means of the several sets represent constant realities, such as would come out the same so long as the general conditions of growth remained unaltered. The principal methods that were adopted are easily explained by selecting one of the shorter series of plants, say of Zea mays, for an example.” Zea mays (young plants). ARRANGED IN ORDER OF MAGNITUDE, As recorded by Mr. Darwin. In Separate Pots. In a Single Series. Il. iI. Vis | Vv. VI. VII. Vill. Crossed. | Self-firt.|| Crossed. | Self-fert. || Crossed. | Self-fert. |Difference Inches. Inches. | Inches. | Inches. |} Inches, | Inches. | Inches. 234 410073: oll 234. |, 208 4) 984 | Noga eae 12 203 || 21 20 232) 1 20% dence 21 20 12 173 || 23 20 | —3 22 20. 99 20 291 1-188 | =—34 191 | 183 || 214 | 188 || 22 183 | —33 Big i 08921 291 4) Se | wee | ae ee 152 || 224 | 18 21 173 33 Pot III. 1682} saga 64 20% | 164 | —3% 18 || 203 | 163 492) | 2462) ] 25 162 || 183 | 153 182 | 154 || 98 | 12 152 | +33 8 23 18 12 128 | +08 Cuap. I. INTRODUCTORY REMARKS. 17 “The observations as I received them are shown in columns II. and IIT., where they certainly have no prima fucie appearance of regularity. But as soon as we arrange them in the order of their magnitudes, as in columns IV. and V., the case is materially altered. We now see, with few exceptions, that the largest plant on the crossed side in each pot exceeds the largest plant on the self-fertilised side, that the second exceeds the second, the third the third, and soon. Out of the fifteen cases in the table, there are only two exceptions to this rule. We may therefore confidently affirm that a crossed series will always be found to exceed a self-fertilised series, within the range of the conditions under which the present experiment has been made.’ Pot. | Crossed. | Self-fert. | Diterence. if 187 | 192 | +03 I, 203 19 —13 Ill. 21} 16g ei e449 8 IV, 198 16 | —3§ “ Next as regards the numerical estimate of this excess. The mean values of the several groups are so discordant, as is shown in the table just given, that a fairly precise numerical estimate seems impossible. But the consideration arises, whether the difference between pot and pot may not be of much the samo order of importance as that of the other conditions upon which the growth of the plants has been modified. If so, and only on that condition, it would follow that when all the measure- ments, either of the crossed or the self-fertiliscd plants, were combined into a single series, that series would be statistically regular. The experiment is tried in columns VII. and VIIL., where the regularity is abundantly clear, and justifies us in considering its mean as perfectly reliable. I have protracted these measurements, and revised them in the usual way, by drawing a curve through them with a free hand, but the re- vision barely modifies the means derived from the original observations. In the present, and in nearly all the other cases, the difference between the original and revised means is under 2 per cent. of their value. It is a very remarkable coincidenes 0 18 INTRODUCTORY REMARKS. Cuar. L that in the scven kinds of plants, whose measurements I have examined, the ratio between the heights of the crossed and of the self-fertilised ranges in five cases within very narrow limits. In Zea mays it is as 100 to 84, and in the others it ranges between 100 to 76 and 100 to 86.” «The determination of the variability (measured by what is technically called the ‘probable error’) is a problem of more delicacy than that of determining the means, and I doubt, after making many trials, whether it is possible to derive useful conclusions from these few observations. We ought to have measurements of at least fifty plants in each case, in order to be in a position to deduce fair results. One fact, however, bearing on variability, is very evident in most cases, though not in Zea mays, viz., that the self-fertilised plants include the larger number of exceptionally small specimens, while the crossed are more generally full grown.” “Those groups of cases in which measurements have been made of a few of the tallest plants that grew in rows, each of which contained a multitude of plants, show very clearly that the crossed plants exceed the self-fertilised in height, but they do not tell by inference anything about their respective mean values. If it should happen that a series is known to follow the law of error or any other law, and if the number of indi- viduals in the series is known, it would be always possible to yeconstruct the whole series when a fragment of it has been given. But I find no such method to be applicable in the present case. The doubt as to the number of plants in each row is of minor importance; the real difficulty lies in our ignorance of the precise law followed by the series. The experience of the plants in pots does not help us to determine that law, because the observations of such plants are too few to enable us to lay down more than the middle terms of the series to which they belong with any sort of accuracy, whereas the cases we are now considering refer to one of its extremities. There are other special difficulties which need not be gone into, as the one already mentioned is a complete bar.” Mr. Galton sent me at the same time graphical representations which he had made of the measure- ments, and they evidently form fairly regular curves. He appends the words “very good” to those of Zea and Grar, I. INTRODUCTORY REMARKS. 19 Limnanthes. He also calculated the average height of the crossed and self-fertilised plants in the seven tables by a more correct method than that followed by me, namely, by including the heights, as estimated in accordance with statistical rules, of a few plants which died before they were measured; whereas I merely added up the heights of the survivors, and divided the sum by their number. The difference in our results is in one way highly satisfactory, for the average heights of the self-fertilised plants, as deduced by Mr. Galton, is less than mine in all the cases excepting one, in which our averages are the same; and this shows that I have by no means exaggerated the superiority of the crossed over the self-fertilised plants. After the heights of the crossed and self-fertilised plants had been taken, they were sometimes cut down close to the ground, and an equal number of both weighed. This method of comparison gives very striking results, and I wish that it had been oftener followed. Finally a record was often kept of any marked difference in the rate of germination of the crossed and self-fertilised seeds,—of the relative periods of flowering of the plants raised from them,—and of their productiveness, that is, of the number of seed- capsules which they produced and of the average number of seeds which each capsule contained. When I began my experiments I did not intend to raise crossed and self-fertilised plants for more than a single generation; but as soon as the plants of the first generation were in flower I thought that I would raise one more generation, and acted in the following manner. Several flowers on one or more of the self- fertilised plants were again self-fertilised ; and several] 02 20 INTRODUCTORY REMARKS. Cuar. I, flowers on one or more of the crossed plants were ferti- lised with pollen from another crossed plant of the same lot. Having thus once begun, the same method was followed for as many as ten successive generations with some of the species. The seeds and seedlings were always treated in exactly the same manner as already described. The self-fertilised plants, whether originally descended from one or two mother-plants, were thus in each generation as closely interbred as was possible ; and I could not have improved on my plan. But instead of crossing one of the crossed plants with another crossed plant, I ought to have crossed the self- fertilised plants of each generation with pollen taken from a non-related plant—that is, one belonging to a distinct family or stock of the same species and variety. This was done in several cases as an additional experi- ment, and gave very striking results. But the plan usually followed was to put into competition and compare intercrossed plants, which were almost always the offspring of more or less closely related plants, with the seltf-fertilised plants of each succeeding genera- tion ;—all having been grown under closely similar conditions. I have, however, learnt more by this method of proceeding, which was begun by an oversight and then necessarily followed, than if I had always crossed the self-fertilised plants of each succeeding generation with pollen from a fresh stock, I have said that the crossed plants of the successive © generations were almost always inter-related. When the flowers on an hermaphrodite plant are crossed with pollen taken from a distinct plant, the seedlings thus raised may be considered as hermaphrodite brothers or sisters ; those raised from the same capsule being as close as twins or animals of the same litter. But in one sense the flowers on the same plant are distinct Ouar. I, INTRODUCTORY REMARKS. plat individuals, and as several flowers on the mother-plant were crossed by pollen taken from several flowers on the father-plant, such seedlings would be in one sense half-brothers or sisters, but more closely related than are the half-brothers and sisters of ordinary animals. The flowers on the mother-plant were, however, com- monly crossed by pollen taken from two or more dis- tinct plants; and in these cases the seedlings might be called with more truth half-brothers or sisters. When two or three mother-plants were crossed, as often happened, by pollen taken from two or three father- plants (the seeds being all intermingled), some of the seedlings of the first generation would be in no way related, whilst many others would be whole or hallf- brothers and sisters. In the second generation a large number of the seedlings would be what may be called whole or half first-cousins, mingled with whole and half-brothers and sisters, and with some plants not at all related. So it would be in the succeeding genera- tions, but there would also be many cousins of the second and more remote degrees. The relationship will thus have become more and more inextricably complex in the later generations; with most of the plants in some degree and many of them closely related. I have only one other point to notice, but this is one of the highest importance; namely, that the crossed and self-fertilised plants were subjected in the same generation to as nearly similar and uniform conditions as was possible. In the successive generations they were exposed to slightly different conditions as the seasons varied, and they were raised at different periods. But in other respects all were treated alike, being grown in pots in the same artificially prepared soil, being watered at the same time, and kept close together in the same greenhouse or hothouse. They were 22. INTRODUCTORY REMARKS. Caar. I. therefore not exposed during successive years to such great vicissitudes of climate as are plants growing out of doors. On some apparent and real Causes of Error in my Ha- periments.—It has been objected to such experiments as mine, that covering plants with a net, although only for a short time whilst in flower, may affect their health and fertility. J have seen no such effect except in one instance with a Myosotis, and the covermg may not then have been the real cause of injury. But even if the net were slightly injurious, and certainly it was not so in any high degree, as I could judge by the appear- ance of the plants and by comparing their fertility with that of neighbouring uncovered plants, it would not have vitiated my experiments; for in all the more im- portant cases the flowers were crossed as well as self- fertilised under a net, so that they were treated in this respect exactly alike. As it is impossible to exclude such minute pollen- carrying insects as Thrips, flowers which it was intended to fertilise with their own pollen may sometimes have been afterwards crossed with pollen brought by these insects from another flower on the same plant; but as we shall hereafter see, a cross of this kind does not produce any effect, or at most only aslight one. When two or more plants were placed near one another under the same net, as was often done, there is some real though not great danger of the flowers which were believed to be self-fertilised being afterwards crossed with pollen brought by Thrips from a distinct plant. I have said that the danger is not great, _ because I have often found that plants which are self-sterile, unless aided by insects, remained sterile when several plants of the same species were placed Cuayp. I. INTRODUCTORY REMARKS. 23 under the same net. If, however, the flowers which had been presumably self-fertilised by me were in any case afterwards crossed by Thrips with pollen brought from a distinct plant, crossed seedlings would have been included amongst the self-fertilised ; but it should be especially observed that this occurrence would tend to diminish and not to increase any superiority in average height, fertility, &c., of the crossed over the self-fertilised plants. As the flowers which were crossed were never cas- trated, it is probable or even almost certain that 1 sometimes failed to cross-fertilise them effectually, and that they were afterwards spontaneously self-fertilised. This would have been most likely to occur with dicho- gamous species, for without much care it is not easy to perceive whether their stigmas are ready to be fer- tilised when the anthers open. But in all cases, as the flowers were protected from wind, rain, and the access of insects, any pollen placed by me on the stigmatic surface whilst it was immature, would gener- ally have remained there until the stigma was mature ; and the flowers would then have’ been crossed as was intended. Nevertheless, it is highly probable that self-fertilised seedlings have sometimes by this means got included amongst the crossed seedlings. The effect would be, as in the former case, not to exaggerate but to diminish any average superiority of the crossed over the self-fertilised plants. Errors arising from the two causes just named, and from others,—such as some of the seeds not having been thoroughly ripened, though care was taken to avoid this error—the sickness or unperceived injury of any of the plants,—will have been to a large extent eliminated, in those cases in which many crossed and self-fertilised plants were measured and an average 24 INTRODUCTORY REMARKS. Cuar. [, struck. Some of these causes of error will also have been eliminated by the seeds having been allowed to germinate on bare damp sand, and being planted in pairs; for it is not likely that ill-matured and well- matured, or diseased and healthy seeds, would germi- nate at exactly the same time. The same result will have been gained in the several cases in which only a few of the tallest, finest, and healthiest plants on each. side of the pots were measured. Kélreuter and Gartner* have proved that with some plants several, even as many as from fifty to sixty, pollen-grains are necessary for the fertilisation of all the ovules in the ovarium. Naudin also found in the case of Mirabilis that if only one or two of its very large pollen-grains were placed on the stigma, the plants raised from such seeds were dwarfed. I was therefore careful to give an amply sufficient supply of pollen, and generally covered the stigma with it; but I did not take any special pains to place exactly the same amount on the stigmas of the self- fertilised and crossed flowers. After having acted in this manner during two seasons, I remembered that Gartner thought, though without any direct evidence, that an excess of pollen was perhaps injurious; and it has been proved by Spallanzani, Quatrefages, and Newport, that with various animals an excess of the seminal fluid entirely prevents fertilisation. It was therefore necessary to ascertain whether the fertility of the flowers was affected by applying a rather small and an extremely large quantity of pollen to the stigma. Accordingly a very small mass of pollen-grains was * ‘Kenntniss der Befruch- tom. i. p. 27. tung,’ 1844, p. 345. Naudin, + ‘Transactions Philosophical ‘Nouvelles Archives du Muséum,’ Soe,’ 1853, pp, 253-258, Cuar. I. INTRODUCTORY REMARKS. 25 placed on one side of the large stigma in sixty-four flowers of Ipomeea purpurea, and a great mass of pollen over the whole surface of the stigma in sixty-four other flowers. In order to vary the experiment, half the flowers of both lots were on plants produced from self- fertilised seeds, and the other half on plants from crossed seeds. ‘The sixty-four flowers with an excess of pollen yielded sixty-one capsules; and excluding four capsules, each of wkich contained only a single poor seed, the remainder contained on an average 5°07 seeds per capsule. The sixty-four flowers with only a little pollen placed on one side of the stigma yielded sixty-three capsules, and excluding one from the same cause as before, the remainder contained on an average 5°129 seeds. So that the flowers fertilised with little pollen yielded rather more capsules and seeds than did those fertilised with an excess; but the difference is too slight to be of any significance. On the other hand, the seeds produced by the flowers with an excess of pollen were a little heavier of the two; for 170 of them weighed 79°67 grains, whilst 170 seeds from the flowers with very little pollen weighed 79°20 grains. Both lots of seeds having been placed on damp sand presented no difference in their rate of germination. We may therefore conclude that my experiments were not affected by any slight difference in the amount of pollen used ; a sufficiency having been employed in all cases. The order in which our subject will be treated in the present volume is as follows. A long series of ex- periments will first be given in Chapters II. to VI. Tables will afterwards be appended, showing in a con- densed form the relative heights, weights, and fertility ot the offspring of the various crossed and self-fertilised 26 INTRODUCTORY REMARKS, Cuar. f, species. Another table exhibits the striking results from fertilising plants, which during several generations had either been self-fertilised or had been crossed with plants kept all the time under closely similar conditions, with pollen taken from plants of a distinct stock and which had been exposed to different con- ditions. In the concluding chapters various related points and questions of general interest will be discussed. Anyone not specially interested in the subject need not attempt to read all the details; though they possess, | think, some value, and cannot be all sum- marised. But I would suggest to the reader to take as an example the experiments on Ipomeea in Chapter II.; to which may be added those on Digitalis, Origa- num, Viola, or the common cabbage, as in all these -eases the crossed plants are superior to the self- fertilised in a marked degree, but not in quite the same manner. As instances of self-fertilised plants being equal or superior to the crossed, the experiments on Bartonia, Canna, and the common pea ought to be read; but in the last case, and probably in that of Canna, the want of any superiority in the crossed plants can be explained. Species were selected for experiment belonging to widely distinct families, inhabiting various countries. In some few cases several genera belonging to the same family were tried, and these are grouped toge- ther; but the families themselves have been arranged not in any natural order, but in that which was the most convenient for my purpose. The experiments have been fully given, as the results appear to me of sufficient value to justify the details. Plants bearing hermaphrodite flowers can be interbred more closely than is possible with the higher animals, and are there- Cuar. L. INTRODUCTORY REMARKS. 27 fore well-fitted to throw light on the nature and extent of the good effects of crossing, and on the evil effects of close interbreeding or self-fertilisation. The most important conclusion at which I have arrived is that the mere act of crossing by itself does no good. The good depends on the individuals which are crossed differing slightly in constitution, owing to their pro- genitors having been subjected during several genera- tions to slightly different conditions, or to. what we call in our ignorance spontaneous variation. This conclusion, as we shall hereafter see, is closely con- nected with various important physiological problems, such as the benefit derived from slight changes in the © conditions of life, and this stands in the closest con- nection with life itself. It throws light on the origin of the two sexes and on their separation or union in the same individual, and lastly on the whole subject of hybridism, which is one of the greatest obstacles to the general acceptance and progress of the great principle of evolution. In order to avoid misapprehension, I beg leave to repeat that throughout this volume a crossed plant, seedling, or seed, means one of crossed parentage, that is, one derived from a flower fertilised with pollen from a distinct plant of the same species. And that a self-fertilised plant, seedling, or seed, means one of self-fertilised parentage, that is, one derived from a flower fertilised with pollen from the same flower, or sometimes, when thus stated, from another flower on the same plant. 28 » IPOM@A PURPUREA. Cuap. IL, CHAPTER II. CONVOLYULACEZ. Ipomoea purpurea, comparison of the height and fertility of tha crossed and self-fertilised plants during ten successive generations —Greater constitutional vigour of the crossed plants—The effects on the offspring of crossing different flowers on the same plant, instead of crossing distinct individuals—The effects of a cross with a fresh stock—The descendants of the self-fertilised plant named Hero—Summary on the growth, vigour, and fertility of the suc- cessive crossed and self-fertilised generations—Small amount of pollen in the anthers of the self-fertilised plants of the later genera- tions, and the sterility of their first-produced flowers—Uniform colour of the flowers produced by the self-fertilised plants—The advantage froma cross between two distinct plants depends on their differing in constitution. A PLANT of Ipomea purpurea, or as it is often called in England the convolyulus major, a native of South America, grew in my greenhouse. ‘Ten flowers on this plant were fertilised with pollen from the same flower ; and ten other flowers on the same plant were crossed with pollen from a distinct plant. The fertilisation of the flowers with their own pollen was superfluous, as this convolvulus is highly self-fertile; but I acted in this manner to make the experiments correspond in all respects. Whilst the flowers are young the stigma projects beyond the anthers; and it might have been thought that it could not be fertilised without the aid of humble-bees, which often visit the flowers; but as the flower grows older the stamens increase in length, and their anthers brush against the stigma, which thus OE ———— Crap. II. CROSSED AND SELF-FERTILISED PLANTS. 29 receives some pollen, The number of seeds produced by the crossed and self-fertilised flowers differed very little. Crossed and self-fertilised seeds obtained in the above manner were allowed to germinate on damp sand, and as often as pairs germinated at the same time they were planted in the manner described in the Introduction, on the opposite sides of two pots. Five pairs were thus planted ; and all the remaining seeds, whether or not in a state of germination, were planted on the opposite sides of a third pot, so that the young plants on both sides were here greatly crowded and exposed to very severe competition. Rods of iron or wood of equal diameter were given to all the plants to twine up; and as soon as one of each pair reached the summit both were measured. A single rod was placed on each side of the crowded pot, No. IIL, and only the tallest plant on each side was measured. TABLE I. (Pirst Generation.) us Seedlings from Seedlings from No. of Pot. Crossed Plants. _| Self-fertilised Plants. Inches, | Inches, 1G 875 69 87 3 66 ! 89 73 10 88 68 3 87 60% A 77 57 Plants crowded; the tallest one mea- sured on each side. Total in inches. 516 394 The average height of the six crossed plants is here 86 inches, whilst that of the six self-fertilised plants is only 65°66 inches, so that the crossed plants are to the self-fertilised in height as 100 to 76. It should be observed that this difference is not due to a few of the crossed plants being extremely tall, or to a few of the self-fertilised being extremely short, but to all the crossed plants attaining a greater height than their antagonists. ‘The three pairs in Pot I. were measured at two earlier periods, and the difference was sometimes greater and sometimes less than that 8c IPOM@A PURPUREA, Cuap. II, at the final measuring. But it is an interesting fact, of which I have seen several other instances, that one of the self-fertilised plants, when nearly a foot in height, was half an inch taller than the crossed plant; and again, when two feet high, it was ‘18 inch taller, but during the ten subsequent days the crossed plant began to gain on its antagonist, and ever afterward asserted its supremacy, until it exceeded its self-fertilised opponent by 16 inches. The five crossed plants in Pots I. and II. were covered with a net, and produced 121 capsules; the five self-fertilised plants produced eighty-four capsules, so that the numbers of capsules were as 100 to 69. Of the 121 capsules on the crossed plants sixty-five were the product of flowers crossed with pollen from a distinct plant, and these contained on an average 5°23 seeds per capsule; the remaining fifty-six capsules were spontaneously self-fertilised. Of the eighty-four capsules on the self-fertilised plants, all the product of renewed self-fertilisation, fifty-five (which were alone examined) contained on an average 4°85 seeds per capsule. Therefore the cross-fertilised capsules, com- pared with the self-fertilised capsules, yielded seeds in the proportion of 100 to 93. The crossed seeds were relatively heavier than the self-fertilised seeds. Combining the above data (i.e., number of capsules and average number of contained seeds), the crossed plants, compared with the self-fertilised, yielded seeds in the ratio of 100 to 64. These crossed plants produced, as already stated, fifty-six spontaneously self-fertilised capsules, and the self-fertilised plants produced twenty-nine such capsules. The former con- tained on an average, in comparison with the latter, seeds in the proportion of 100 to 99. In Pot ILL, on the opposite sides of which a large number of crossed and self-fertilised seeds had been sown and the seed- lings allowed to struggle together, the crossed plants had at first no great advantage. At one time the tallest crossed was 25% inches high, and the tallest self-fertilised plants 213. But the difference afterwards became much greater. The plants on both sides, from being so crowded, were poor specimens. The flowers were allowed to fertilise themselves spontaneously under a net; the crossed.plants produced thirty-seven capsules, the self-fertilised plants only eighteen, or as 100 to 47. The former contained on an average 3°62 seeds per capsule; and the latter 3°88 seeds, or as 100 to 93, Combining these data (i.¢., number * Cuar. II. CROSSED AND SELF-FERTILISED PLANTS. ol of capsules and average number of seeds), the crowded crossed plants produced seeds compared with the self-fertilised as 100 to 45, These latter seeds, however, were decidedly heavier, a hundred weighing 41°64 grains, than those from the capsules on the crossed plants, of which a hundred weighed 36°79 grains ; and this probably was due to the fewer capsules borne by the self-fertilised plants having been better nourished. We thus see that the crossed plants in this the first generation, whén grown under favourable conditions, and when grown under unfavour- able conditions from being much crowded, greatly exceeded in height, and in the number of capsules produced, and slightly in the number of seeds per capsule, the self-fertilised plants. Crossed and self-fertilised Plants of the Second Generation.— Flowers on the crossed plants of the last generation (Table I.) were crossed by pollen from distinct plants of the same genera- tion; and flowers on the self-fertilised plants were fertilised by pollen from the same flower. ‘The seeds thus produced were treated in every respect as before, and we have in Table II. the result. Taste IT, (Second Generation.) No. of Pot. | Crossed Plants. Self-fertilised Plants. Inches, als Inches. 87 67 3 §3 683 83 803 85 Gyadebt tranny 61 i 89 49 T7 4 41 Total inches. Total inches. | 505 505 598 Here again every single crossed plant is taller than its anta- gonist. The self-fertilised plant in Pot I., which ultimately reached the unusual height of 804 inches, was for a long time taller than the opposed crossed plant, though at last beaten by it. The average height of the six crossed plants is 84:16 inches, whilst that of the six self-fertilised plants is 66°33 inches, or as 100 to 79. Crossed and self-fertilised Plants of the Third Generation —Seeds from the crossed plants of the last generation (Table IT.) agaiu oo IPOMG@A PURPUREA. Cuaap. II, crossed, and from the self-fertilised plants again self-fertilised, were treated in all respects exactly as before, with the following result :— Tanne LI. (Third Generation.) Apap eee inches. EG Mgeaca eins 5 317°0 No. of Pot. | Crossed Plants. Self-fertilised Plants. | ee Inches. 74 56 4 72 51 5 1(3 4 54 ean aa 82 59 81 30 Ee See 66 Again all the crossed plants are higher than their antagonists: their average height is 77°41 inches, whereas that of the self- fertilised is 52°83 inches, or as 100 to 68. I attended closely to the fertility of the plants of this third generation. Thirty flowers on the crossed plants were crossed with pollen from other crossed plants of the same generation, and the twenty-six capsules thus produced contained, on an average, 4°73 seeds; whilst thirty flowers on the self-fertilised plants, fertilised with the pollen from the same flower, produced twenty-three capsules, each containing 4°43 seeds. Thus the average number of seeds in the crossed capsules was to that in the self-fertilised capsules as 100 to 94. A hundred of the crossed seeds weighed 43°27 grains, whilst a hundred of the self- fertilised seeds weighed only 37°63 grains. Many of these lighter self-fertilised seeds placed on damp sand germinated before the crossed; thus thirty-six of the former germinated whilst only thirteen of the latter or crossed seeds germinated. In Pot I. the three crossed plants produced spontaneously under the net (besides the twenty-six artificially cross-fertilised capsules) seventy-seven self-fertilised capsules containing on an average 4°41 seeds; whilst the three self-fertilised plants produced spontaneously (besides the twenty-three artificially self-fertilised capsules) only twenty-nine self-fertilised capsules, containing on an average 4°14 seeds. Therefore the average number of seeds in the two lots af spontaneously self-fertilised capsules was as Cnar. II, CROSSED AND SELF-FERTILISED PLANTS. 383 100 to 94. Taking into consideration the number of capsules together with the average number of seeds, the crossed plants (spontaneously self-fertilised) produced seeds in comparison with the self-fertilised plants (spontaneously self-fertilised) in the proportion of 100 to 35. By whatever method the fertility of these plants is compared, the crossed are more fertile than the self-fertilised plants. I tried in several ways the comparative vigour and powers of growth of the crossed and self-fertilised plants of this third generation. Thus, four self-fertilised seeds which had just germinated were planted on one side of a pot, and after an in- terval of forty-eight hours, four crossed seeds in the same state of germination were planted on the opposite side; and the pot was kept in the hothouse. I thought that the advantage thus given to the self-fertilised seedlings would have been so great that they would never have been beaten by the crossed ones. They were not beaten until all had grown to a height of 18 inches; and the degree to which they were finally beaten is shown in the following table (No. IV.). We here see that the average height of the four crossed plants is 76°62, and of the four self-fertilised plants 65°87 inches, or as 100 to 86; there- fore less than when both sides started fair. Taste LV. (Third Generation, the self-fertilised Plants having had a start of forty-eight hours.) No. of Pot. Crossed Plants. Self-fertilised Plants, Inches, Inches, III. 784 13 4 774 53 73 413 | T7 4 754 Total inches. | 306°5 263°5 Crossed and self-fertilised seeds of the third generation were also sown out of doors late in the summer, and therefore under unfavourable conditions, and a single stick was given 1o each lot of plants to twine up. The two lots were sufficiently separate so as not to interfere with each other’s growth, and the ground was clear of weeds. As soon as they were killed by the first frost (and there was no difference in their hardiness), the two tallest crossed plants were found to be 24°5 and 22°5 inches, D 34 IPOMGA PURPUREA. Cuar. IL, whilst the two tallest self-fertilised plants were only 15 and 12-5 inches in height, or as 100 to 59. I likewise sowed at the same time two lots of the same seeds in a part of the garden which was shady and covered with weeds. The crossed seedlings from the first looked the most healthy, but they twined up a stick only to a height of 7t inches; whilst the self-fertilised were not able to twine at all; and the tallest of them was only 33 inches in height. Lastly, two lots of the same seeds were sown in the midst of a bed of candy-tuft (Iberis) growing vigorously. The seedlings came up, but all the self-fertilised ones soon died excepting one, which never twined and grew to a height of only 4 inches. Many of the crossed seedlings, on the other hand, survived; and some twined up the stems of the Iberis to the height of 11 inches. These cases prove that the crossed seedlings have an immense advantage over the self-fertilised, both when growing isolated under very unfavourable conditions, and when put into competition with each other or with other plants, as would happen in a state of nature. Crossed and self-fertilised Plants of the Fourth Generation.—Seed- lings raised as before from the crossed and self-fertilised plants ot the third generation in Table III., gave results as follows :— TABLE V. (Fourth Generation.) | No. of Pot. Crossed Plants. | Self-fertilised Plants. Inches. i Inches. if 84 80 | 47 443 | IL, | 83 734 59 514 III. 82 563 654 63 68 52 Total inches! | 488°5 421°0 Here the average height of the seven crossed plants is 69°78 inches, and that of the seven self-fertilised plants 60°14; or as 100 to 86. This smaller difference relatively to that in the former generations, may be attributed to the plants having been raised during the depth of winter, and consequently to their not Cuar. II. CROSSED AND SELF-FERTILISED PLANTS. 39 having grown vigorously, as was shown by their general ap- pearance and from several of them neyer reaching the summits’ of the rods. In Pot II., one of the self-fertilised plants was for a long time taller by two inches than its opponent, but was ultimately beaten by it, so that all the crossed plants exceeded their opponents in height. Of twenty-eight capsules produced by the crossed plants fertilised by pollen from a distinct plant, each contained on an average 4°75 seeds; of twenty-seven self- fertilised capsules on the self-fertilised plants, each contained on an average 4°47 seeds; so that the proportion of seeds in the crossed and self-fertilised capsules was as 100 to 94. Some of the same seeds, from which the plants in the last Table V. had been raised, were planted, after they had germi- nated on damp sand, in a square tub, in which a large Brug- mansia had long been growing. The soil was extremely poor and full of roots; six crossed seeds were planted in one corner, and six self-fertilised seeds in the opposite corner. All the seedlings from the latter soon died excepting one, and this grew to the height of only 13 inch. Of the crossed plants three survived, and they grew to the height of 23 inches, but were not able to twine round a stick; nevertheless, to my surprise, they produced some small miserable flowers. The crossed plants thus had a decided advantage over the self-fertilised plants under this extremity of bad conditions. Crossed und self-fertilised Plants of the Fifth Generation.—Theso were raised in the same manner as before, and Tae measured gave the following results :-— TaBLe VI. (Lifth Generation.) Seat RTO wat Date dind tive Bal Wn ea Vie Dae bn a No. of Pot. motto | crema Panta, Crossed Plants, Self-fertilised Plants. Inches. Inches, 96 73 86 78 69 29 84 ol 84 84 764 | 59 - Total inches. 495:+25 | 374-00 The average height of the six crossed plants is 82°54 inches, D2 26 IPOM@A PURPUREA. Cuar. II, and that of the six self-fertilised plants 62°33 inches, or as 100 to 75. Every crossed plant exceeded its antagonist in height. In Pot I. the middle plant on the crossed side was slightly injured whilst young by a blow, and was for a time beaten by its opponent, but ultimately recovered the usual superiority. The crossed plants produced spontaneously a vast number more capsules than did the self-fertilised plants; and the capsules of the former contained on an average 3°87 seeds, whilst those of the latter contained only 3-0 per capsule, or as 100 to 89. But looking only to the artificially fertilised capsules, those on the crossed plants again crossed contained on an average 4°46 seeds, whilst those on the self-fertilised plants again self- fertilised contained 4°77 seeds; so that the self-fertilised cap- sules were the more fertile of the two, and of this unusual fact I can offer no explanation. Crossed and self-fertilised Plants of the Sixth Generation.— These were raised in the usual manner, with the following result. I should state that there were originally eight plants on each side; but as two of the self-fertilised became extremely un- healthy and never grew to near their full height, these as well as their opponents have been struck out of the list. If they had been retained, they would have made the average height of the crossed plants unfairly greater than that of the self-fertilised. I have acted in the same manner in a few other instances, when one of a.pair plainly became very unhealthy. TABLE VII. (Stxth Generation.) No, of Pot, Crossed Plants. Self-fertilised Plants. Inches. Inches, i: 93 504 91 65 II 73) 50 864 87 88 62 Ti 874 644 ‘Total inches, 525 379 The average height of the six crossed plants is here $7°5, and of the six self-fertilised plants 63°16, oras 100 to 72. This large difference was chiefly due to most of the plants, especially the * Cuar. 11, CROSSED AND SELF-FERTILISED PLANTS. 3? self-fertilised ones, having become unhealthy towards the close of their growth, and they were severely attacked by aphides. From this cause nothing can be inferred with respect to their relative fertility. In this generation we have the first instance of a self-fertilised plant in Pot II. exceeding (though only by half an inch) its crossed opponent. This victory was fairly won after a long struggle. At first the self-fertilised plant was several inches taller than its opponent, but when the latter was 4% feet high it had grown equal; it then grew a little taller than the self-fertilised plant, but was ultimately beaten by it to the extent of half an inch, as shown in the table. I was so much surprised at this case that I saved the self-fertilised seeds of this plant, which I will call the “ Hero,” and experimented on its descendants, as will hereafter be described. Besides the plants included in Table VII., nine crossed and nine self-fertilised plants of the same lot were raised in two other pots, IV. and VY. These pots had been kept in the hot- house, but from want of room were, whilst the plants were young, suddenly moved during very cold weather into the coldest part of the greenhouse. They all suffered greatly, and never quite recovered. After a fortnight only two of the nine self-fertilised seedlings were alive, whilst seven of the crossed survived. The tallest of these latter plants when measured was 47 inches in height, whilst the tallest of the two surviving self- fertilised plants was only 32 inches. Here again we see how much more vigorous the crossed plants are than the self-fertilised. Crossed and self-fertilised Plants of the Seventh Generation. These were raised as heretofore with the following result :— TaBLE VIII. (Seventh Generation.) | Self-fertilised Plants. No. of Pot. | Crossed Plants. Naa bribe ees { Inches. Inches, ie 844 748 848 84 763 554 II 844 65 90 513 a 823 804 lll 83 | 67§ 86 603 IV. 2 843 753 Total inches. 755°50 614°25 Pete. IPOMG@A PURPUREA Cuar. IL. Each of these nine crossed plants is higher than its opponent, though in one case only by three-quarters of an inch. Their average height is 83°94 inches, and that of the self-fertilised plants 68°25, or as 100 to 81. These plants, after growing to their full height, became very unhealthy and infested with aphides, just when the seeds were setting, so that many of the capsules failed, and nothing can be said on their relative fertility. Crossed and self-fertilised Plants of the Eighth Generation —As just stated, the plants of the last generation, from which the present ones were raised, were very unhealthy and their seeds of unusually small size; and this probably accounts, through abnormal premature growth, for the two lots behaving dif- ferently to what they did in any of the previous or succeeding generations. Many of the self-fertilised seeds germinated before the crossed ones, and these were of course rejected. When the crossed seedlings in Table IX. had grown to a height of between L and 2 feet, they were all, or almost all, shorter than their self- fertilised opponents, but were not then measured. "When they had acquired an average height of 82:28 inches, that of the self-fertilised plants was 40°68, or as 100 to 122. Moreover, every one of the self-fertilised plants, with a single exception, exceeded its crossed opponent. When, however, the crossed plants had grown to an average height of 77°56 inches, they just exceeded (viz., by 7 of an inch) the average height of the self-fertilised plants; but two of the latter were still taller than ° their crossed opponents. I was so much astonished at this whole case, that I tied string to the summits of the rods; the plants being thus allowed to continue climbing upwards. When their growth was complete they were untwined, stretched straight, and measured. The crossed plants had now almost regained their accustomed superiority, as may be seen in Table IX. The average height of the eight crossed plants is here 118°25 inches, and that of the self-fertilised plants 96°65, or as 100 te 85. Nevertheless two of the self-fertilised plants, as may be seen in the table, were still higher than their crossed opponents. The latter manifestly had much thicker stems and many more lateral branches, and looked altogether more vigorous than the self-fertilised plants, and generally flowered before them. The earlier flowers produced by these self-fertilised plants did not set any capsules, and their anthers contained only a small amount of pollen; but to this subject I shall return. Neverthe Cuar. If. CROSSED AND SELF-FERTILISED PLANTS. 939 less capsules produced by two other self-fertilised plants of the same lot, not included in Table IX., which had been highly favoured by being grown in separate pots, contained the large average number of 5:1 seeds per capsule. TasLE IX. (Highth Generation.) eee eee of Pot. Crossed Plants. Self-fertilised Plants. Inches. Inches. 1113 96 127 54 1308 934 972 94 894 1258 Il. 1038 1153 1008 848 carom 1473 1098 Total inches. 908°25 973°25 Crossed and self-fertilised Plants of the Ninth Generation.— The plants of this generation were raised in the same manner as before, with the result shown in Table X. The fourteen crossed plants average in height 81°39 inches and the fourteen self-fertilised plants 64°07, or as 100 to 79. One self-fertilised plant in Pot III. exceeded, and one in Pot IV. equalled in height, its opponent. ‘The self-fertilised plants showed no sign of inheriting the precocious growth of their parents; this having been due, as it would appear, to the abnormal state of the seeds from the unhealthiness of their parents. ‘The fourteen self-fertilised plants yielded only forty spontaneously self-fertilised capsules, to which must be added seven, the product of ten flowers artificially self-fertilised. On the other hand, the fourteen crossed plants yielded 152 spon- taneously self-fertilised capsules; but thirty-six flowers on these plants were crossed (yielding thirty-three capsules), and these flowers would probably have produced about thirty sponta- neously self-fertilised capsules. Therefore an equal number of the crossed and self-fertilised plants would have produced capsules in the proportion of about 182 to 47, or as 100 to 26. Another phenomenon was well pronounced in this generation, but I believe had occurred previously to a slight extent ; 40 IPOM@A PURPUREA. Cuar. II. ‘namely, that most of the flowers on the self-fertilised plants were somewhat monstrous. The monstrosity consisted in the corolla being irregularly split so that it did not open properly, with one or two of the stamens slightly foliaceous, coloured, and firmly coherent to the corolla. I observed this monstrosity in only one flower on the crossed plants. The self-fertilised plants, if well nourished, would almost certainly, in a few more generations, have produced double flowers, for they had already become in some degree sterile.* TABLE X. (Ninth Generation.) No. of Pot. Crossed Plants. Self-fertilised Plants. Inches, Inches. 18 834 57 854 71 834 483 Ii. 83 3 45 645 436 645 383 Ill. 79 63 884 G1 61 893 IV 82 3 82 4 90 765 V. 894 67 Crowded plants. 924 742 94 3 924 70 Total inches. 1139°5 897°0 Crossed and self-fertilised Plants of the Tenth Generation.—Six plants were raised in the usual manner from the crossed plants of the last generation (Table X.) again intercrossed, and from the self-fertilised again self-fertilised. As one of the crossed plants in Pot I. in the following table became much diseased, having crumpled leaves, and producing hardly any capsules, it and its opponent have been struck out of the table, ~ * See on this subject‘ Variation | Domestication, chap. xviii, 2nd of Animals and ‘Planta under edit, yol. if. p.152, " Cuar. II. FLOWERS ON SAME PLANT CROSSED. 41 TABLE XI. (Tenth Generation.) No. of Pot. Crossed Plants. Self-fertilised Plants. ; ee Inches Inches. Is 923 473 94% 348 Il. 87 543 893 493 105 663 Total inches, 468°5 | 252':0 The five crossed plants average 93°7 inches, and the five self- fertilised only 50°4, or as 100 to 54. This difference, however, is so great that it must be looked at as in part accidental. The six crossed plants (the diseased one here included) yielded spon- taneously 101 capsules, and the six self-fertilised plants 88, the latter being chiefly produced by one of the plants. But as the diseased plant, which yielded hardly any seed, is here included, the ratio of 101 to 88 does not fairly give the relative fertility of the two lots. The stems of the six crossed plants looked so much finer than those of the six self-fertilised plants, that after the capsules had been gathered and most of the leaves had fallen off, they were weighed. Those of the crossed plants weighed 2,693 grains, whilst those of the self-fertilised plants weighed only 1,173 grains, or as 100 to 44; but as the diseased and dwarfed crossed plant is here included, the superiority of the former in weight was really greater, The Effects on the Offspring of crossing different Flowers on the same Plant, instead of crossing distinct Individuals. —In all the foregoing experiments, seedlings from flowers crossed by pollen from a distinct plant (though in the later generations more or less closely related) were put into competition with, and almost invariably proved markedly superior in height to the offspring from self-fertilised flowers. I wished, therefore, to ascertain whether a cross between two flowers on the same plant would give to the offspring any superiority 4B.) IPOMCA PURPUREA. Cuap. {1 over the offspring from flowers fertilised with their own pollen. I procured some fresh seed and raised two plants, which were covered with a net; and several of their flowers were crossed with pollen from a dis- tinct flower on the same plant. ‘l'wenty-nine capsules thus produced contained on an average 4°86 seeds per capsule ; and 100 of these seeds weighed 86°77 grains. Several other flowers were fertilised with their own pollen, and twenty-six capsules thus produced con- tained on an average 4°42 seeds per capsule; 100 of which weighed 42°61 grains. So that a cross of this kind appears to have increased slightly the number of seeds per capsule, in the ratio of 100 to 91; but these crossed seeds were lighter than the self-fertilised in the ratio of 86 to100. I doubt, however, from other observations, whether these results are fully trust- worthy. The two lots of seeds, after germinating on sand, were planted in pairs on the opposite sides of nine pots, and were treated in every respect like the plants in the previous experiments. The remaining seeds, some in a state of germination and some. not so, were sown on the opposite sides of a large pot (No. X.); and the four tallest plants on each side of this pot were measured. The result is shown in Table XII. The average height of the thirty-one crossed plants is 73°23 inches, and that of the thirty-one self-fertilised plants 77°41 inches; or as 100 to 106. Looking to each pair, it may be seen that only thirteen of the crossed plants, whilst eighteen of the self-fertilised plants exceed their opponents. A record was kept with respect to the plant which flowered first in each pot ; and only two of the crossed flowered before one of the self-fertilised in the same pot; whilst eight of the self- fertilised flowered first. It thus appears that the Ouar. IT. FLOWERS ON SAME PLANT CROSSED. 43 TABLE XII. No. of Pet. Crossed Plants. Self-fertilised Plants, Inches, Inches, I 82 773 75 87 65 64 76 872 IL, 783 84 43 864 653 908 Uf. 612 86 85 693 89 875 a H — Vic 83 803 73% 88 § 67 843 Vv. 78 663 768 773 57 Sis VI. 70§ 80 79 823 798 554 VII. 76 vers 84! 833 aie 735 VIL. 73 765 67 82 83 80, 1X 733 784 78 67§ dG 34 823 Crowded plants, 82 368 846 694 (ial 752 Fh ee ma fie we ty sh Se Totai inches. 2270°25 2399°75 ch NE STEELS EE ET es Memes ec We OE 2 th ee 44 POMG@A PURPUREA. Cuapr. IL crossed plants are slightly inferior in height and in earliness of flowering to the self-fertilised. But the inferiority in height is so small, namely as 100 to 106, that I should have felt very doubtful on this head, had I not cut down all the plants (except those in the crowded pot No. X.) close to the ground and weighed them. ‘The twenty-seven crossed plants weighed 164 ounces, and the twenty-seven self-fer- tilised plants 20$ ounces; and this gives a ratio of 100 to 124. A self-fertilised plant of the same parentage as those in Table XII. had been raised in a separate pot for a distinct purpose; and it proved partially sterile, the anthers containing very little pollen. Several flowers on this plant were crossed with the little pollen which could be obtained from the other flowers on the same plant ; and other flowers were self-fertilised. J’rom the seeds thus produced four crossed and four self-fertilised plants were raised, which were planted in the usual manner on the opposite sides of two pots. All these four crossed plants were inferior in height to their opponents; they averaged 78°18 inches, whilst the four self-fertilised plants averaged 84:8 inches ; or as 100 to 108.* ‘This case, therefore, confirms the last. Taking all the evidence together, we must conclude that these strictly self-fertilised plants grew a little taller, were heavier, and generally flowered before those derived from a cross between two flowers on the same plant. These latter plants thus present a won- derful contrast with those derived from a cross between two distinct individuals. * From one of these self-ferti- an average only 8°2 seeds per lised plants, spontaneously self- capsule; so that this plant had fertilised, I gathered twenty-four apparently inherited some of the capsules, and they contained on sterility of its parent, Cuap. IT. CROSS WITH A FRESH STOCK. 45 The Effects on the Offspring of a Cross with a distinct or fresh Stock belonging to the same Variety. From the two foregoing series of experiments we see, firstly, the good effects during several successive generations of a cross between distinct plants, although these were in some degree inter-related and had been grown under nearly the same conditions ; and, secondly, the absence of all such good effects from a cross between flowers on the same plant; the comparison in both cases being made with the offspring of flowers fertilised with their own pollen. The experiments now to be given show how powerfully and beneficially plants, which have been intercrossed during many successive generations, having been kept all the time under nearly uniform conditions, are affected by a cross with another plant belonging to the same variety, but to a distinct family or stock, which had grown under dif- ferent conditions. Several flowers on the crossed plants of the ninth generation in Table X., were crossed with pollen from another crossed plant of the same lot. The seedlings thus raised formed the tenth intercrossed generation, and I will call them the ‘ ¢xtercrossed plants.” Several other flowers on the same crossed plants ot the ninth generation were fertilised (not having been castrated) with pollen taken from plants of the same variety, but belonging to a distinct family, which had been grown in a distant garden at Colchester, and therefore under somewhat different conditions. The capsules produced by this cross contained, to my surprise, fewer and lighter seeds than did the capsules of the intercrossed plants; but this, I think, must have been accidental. ‘The seed- lings raised from them I will call the “ Colchester-crossed.” The two lots of seeds, after germinating on sand, were planted in the usual manner on the opposite sides of five pots, and the remaining seeds, whether or not in a state of germination, were thickly sown on the opposite sides of a very large pot, No. VL, in Table XIII. In three of the six pots, after the young plants had twined a short way up their sticks, one of the 46 IPOMG:A PURPUREA. Cuap. IL. Colchester-crossed plants was much taller than any one of the intercrossed plants on the opposite side of the same pot; and in the three other pots somewhat taller. I should state ‘that two of the Colchester-crossed plants in Pot IV., when about two-thirds grown, became much diseased, and were, together with their intercrossed opponents, rejected. The remaining nineteen plants, when almost fully grown, were measured, with the following result :— TABLE XIII. Colchester-crossed jIntercrossed Plants of No. of Pot. Plants, | the Tenth Generation. Inches. | Inches. 78 68! VI. Crowded plants ina very large pot. Total inches. 1596°50 In sixteen out of these nineteen pairs, the Colchester-crossed plant exceeded in height its intercrossed opponent. The average height of the Colchester-crossed is 84°03 inches, and that of the intercrossed 65°78 inches; or as 100 to78. With respect Cuar. I. DESCENDANTS OF HERO. 47 to the fertility of the two lots, it was too troublesome to collect and count the capsules on all the plants; so I selected two of the best pots, V. and VI., and in these the Colchester-crossed produced 269 mature and half-mature capsules, whilst an equal number of the intercrossed plants produced only 154 capsules ; or as 100 to 57. By weight the capsules from the Colchester- crossed plants were to those from the intercrossed plants as 100 to 51; so that the former probably contained a somewhat larger average number of seeds. We learn from this important experiment that plants in some degree related, which had been inter- erossed during the nine previous generations, when they were fertilised with pollen from a fresh stock, yielded seedlings as superior to the seedlings of the tenth intercrossed generation, as these latter were to the self- fertilised plants of the corresponding generation. For if we look to the plants of the ninth generation in Table X. (and these offer in most respects the fairest standard of comparison) we find that the intercrossed plants were in height to the self-fertilised as 100 to 79, and in fertility as 100 to 26; whilst the Colchester- crossed plants are in height to the intercrossed as 100 to 78, and in fertility as 100 to 51. The Descendants of the self-fertilised Plant, named Hero, which appeared in the Sixth self-fertilised Generation.—In the five genera tions before the sixth, the crossed plant of each pair was taller than its self-fertilised opponent; but in the sixth generation (Table VII., Pot II.) the Hero appeared, which after a long and dubious struggle conquered its crossed opponent, though by only half an inch, I was so much surprised at this fact, that I resolved to ascertain whether this plant would transmit its powers of growth to its seedlings. Several flowers on Hero were therefore fertilised with their own pollen, and the seedlings thus raised were put into competition with self-fer- tilised and intercrossed plants of the corresponding generation. The three lots of seedlings thus all belong to the seventh genera- 48 IPOM@A PURPUREA. Cuap. II, tion. Their relative heights are shown in the two following tables :— TABLE XIV. Self-fertilised Plants aie Self-fertilised Plants No. of Pot. cobra evan Gor of the Seventh Gene- a on, ration. Inches. Inches. I. 74 893 60 61 555 49 II 92 82 91 § 56 742 3 Total inches, 447 °25 375°50 The average height of the six self-fertilised children of Hero is 74°54 inches, whilst that of the ordinary self-fertilised plants of the corresponding generation is only 62°58 inches, or as 100 to 84, ; TABLE XY. Self-fertilised Plants : Intercrossed Plants of of the Seventh Gene- No. of Pot. ration, Children of the Seventh Gene- Hero: ration. Inches. Inches. Ill. 92 76 § IV. 87 89 87 g 86 § Total inches. 266°75 252°50 Here the average height of the three self-fertilised children of Hero is 88°91 inches, whilst that of the intercrossed plants is 84°16; or as 100 to 95. We thus see that the self-fertilised children of Hero certainly inherit the powers of growth of their parents; for they greatly exceed in height the self-fertiliscd offspring of the other self-fertilised plants, and even exceed by a trifle the intercrossed plants,—all of the corresponding generation. Cuap. II. DESCENDANTS OF HERO. 49 Several flowers on the self-fertilised children of Heroin Table XIV. were fertilised with pollen from the same flower; and from the seeds thus produced, self-fertilised plants of the eighth generation (grand-children of Hero) were raised. Several other flowers on the same plants were crossed with pollen from the other children of Hero. The seedlings raised from this cross may be considered as the offspring of the union of brothers and sisters. The result of the competition between these two sets of seedlings (namely self-fertilised and the offspring of brothers and sisters) is given in the following table :— TABLE XVI. | Self-fertilised Grand- | Grandchildren from a children of Hero, {cross between the self- No. of Pot. from the Self-fertilised| fertilised Children of Children. Kighth | Hero. Highth Gene- Generation. Tation. Inches. Inches. IE 86 6 95 5 902 953 II $6 85 773 os Ul 73 3 66 825 844 8 IV. 88} 663 84 153 36% 38 74 78 é V 90} 826 90 3 8 Total inches. 1037°00 973-13 The average height of the thirteen self-fertilised grandchildren of Hero is 79°76 inches, and that of the grandchildren from a cross between the self-fertilised children is 74°85; or as 100 to 94. But in Pot IV. one of the crossed plants grew only to a height of 153 inches; and if this plant and its opponent are struck out, as would be the fairest plan, the average height of the crossed plants exceeds, but only by a fraction of an inch, that of the self- E 50 IPOMG@A PURPUREA, Cuar. IL fertilised plants. It is therefore clear that a cross between the self-fertilised children of Hero did not produce any beneficial effect worth notice; and it is very doubtful whether this negative result can be attributed merely to the fact of brothers and sisters having been united, for the ordinary intercrossed plants of the several successive generations must often have been derived from the union of brothers and sisters (as shown-in Chap. I.), and yet all of them were greatly superior to the self-fertilised plants. We are therefore driven to the suspicion, which we shall soon see strengthened, that Hero transmitted to its offspring a peculiar constitution adapted for self-fertilisation. It would appear that the self-fertilised descendants of Hero have not only inherited from Hero a power of growth equal to that of the ordinary intercrossed plants, but have become more fertile when self-fertilised than is usual with the plants of the present species. The flowers on the self-fertilised grandchildren of Hero in Table XVI. (the eighth generation of self-fertilised plants) were fertilised with their own pollen and produced plenty of capsules, ten of which (though this is too few a number for a safe average) contained 5-2 seeds per capsule,—a higher average than was observed in any other case with the self-fertilised plants. The anthers produced by these self-fertilised grandchildren were also as well developed and contained as much pollen as those on the intercrossed plants of the corresponding generation; whereas this was not the case with the ordinary self-fertilised plants of the later generations. Nevertheless some few of the flowers produced by the grandchildren of Hero were slightly monstrous, like those of the ordinary self-fertilised plants of the later genera- tions. In order not to recur to the subject of fertility, I may add that twenty-one self-fertilised capsules, spontaneously produced by the great-grandchildren of Hero (forming the ninth generation of self-fertilised plants), contained on an average 4°47 seeds; and this is as high an average as the self-fertilised flowers of any generation usually yielded. Several flowers on the self-fertilised grandchildren of Hero in Table XVI. were fertilised with pollen from the same flower; and the seedlings raised from them (great-grandchildren of Hero) formed the ninth self-fertilised generation. Several other flowers were crossed with pollen from another grandchild, so that they may be considered as the offspring of brothers and sisters, and the seedlings thus raised may be called the intercrossed great-grand- children. And lastly, other flowers were fertilised with pollen / Cuar. IT. SUMMARY OF OBSERVATIONS. 51 from a distinct stock, and the seedlings thus raised may bo called the Colchester-crossed great-grandchildren. In my anxiety to see what the result would be, I unfortunately planted the three lots of seeds (after they had germinated on sand) in the hothouse in the middle of winter, and in consequence of this the seedlings (twenty in number of each kind) became very unhealthy, some growing only a few inches in height, and very few to their proper height. ‘The result, therefore, cannot be fully trusted ; and it would be useless to give the measurements in detail. In order to strike as fair an average as possible, I first excluded all the plants under 50 inches in height, thus rejecting all the most unhealthy plants. The six self-fertilised thus left were on an average 66°86 inches high; the eight intercrossed plants 63°2 high; and the seven Colchester-crossed 65°37 high; so that there was not much difference between the three sets, the self- fertilised plants having aslight advantage. Nor was there any great difference when only the plants under 36 inches in height were excluded. Nor again when all the plants, however much dwarfed and unhealthy, were included. In this latter case the Colchester-crossed gave the lowest average of all; and if these plants had been in any marked manner superior to the other two lots, as from my former experience I fully expected they would have been, I cannot but think that some vestige of such superiority would have been evident, notwithstanding the very unhealthy condition of most of the plants. Noadvantage, as'far as we can judge, was derived from intercrossing two of the grandchildren of Hero, any more than when two of the children were crossed. It appears therefore that Hero and its descendants have varied from the common type, not only in acquiring great power of growth, and increased fertility when subjected to self- fertilisation, but in not profiting from a cross with a distinct stock ; and this latter fact, if trustworthy, is a unique case, as far as I have observed in all my experiments. Summary on the Growth, Vigour, and Fertility of the successive Generations of the crossed and self-fertilised Plants of Ipomea purpurea, together with some miscel- laneous Observations. In the following table, No. X VIL, we see the average or mean heights of the ten successive generations of the intercrossed and self-fertilised plants, grown in E 2 52 IPOMG@A PURPUREA. Cuap. Ih. competition with each other; and in the right-hand column we have the ratios of the one to the other, the height of the intercrossed plants being taken at 100. In the bottom line the mean height of the seventy- three intercrossed plants is shown to be 85°84 inches, and that of the seventy-three self-fertilised plants 66°02 inches, or as 100 to 77. TaBLeE XVII. Tpomea purpurea, Summary of Measurements (in Inches) of the Ten Generations. Number} Average |Number} Average | Ratio between of Height | of Self- |Height o! Average Number of the Generation. | Crossed jof Crossed} ferti- |Selt-ferti-| Heights of Plants. | Plants.’ | lised lised Crossed and Plants. | Plants. | Self-fertilised Plants. First generation. . . 6 | 86°00 6 | 65°66 Jas 100 to 76 Table I. Second generation . . 6 | 84:16 6 | 66°33 Jas 100 to 79 Table II. Third generation. . . G7 77541 6 | 52°83 jas 100 to 68 Table III. Fourth generation 69°78 | 7 | 60°14 Jas 100 to 86 Table V. Fifth generation. . . | 6 82°54 6 62°33 Jas 100 to 75 Table VI. *16 Jas 100 to 72 (op) iva “I On o for} for} oo Sixth generation . Table VII. Seventh generation . 9 | 83-94 9 | 68°25 jas 100 to 81 Table VIL. Eighth generation . . 8 |113*25 8 | 96°65 jas 100 to 85 - Table 1X. Ninth generation . .| 14 | 81°39| 14 | 64-07 las 100 to 79 Table X. | Tenth generation ., . 5 | 93°70 5 | 50°40 jas 100 to 54 Table XI, All the ten generations oe Meier Jari ni *\ 73 | 85°84 | 73 | 66°02 |as 100 to 77 A ER Rg ©uap., Il, SUMMARY OF OBSERVATIONS. 53 The mean height of the self-fertilised plants in each of the ten generations is also shown in the accompanying diagram, that of the intercrossed plants being taken at 100; and on the right side we see the relative heights of the seventy-three intercrossed plants, and of the seventy-three self-fertilised plants. The difference in height between the crossed and To git gh shh Git 7h git gt igh jean uf the ten Generation Generations Diagram showing the mean heights of the crossed and self-fertilised plants of Jpomea purpurea in the ten generations; the mean height of the crossed plants being taken as 100, On the right hand, the mean heights of the crossed and self-fertilised plants of all the generations taken together are shown, self-fertilised plants will perhaps be best appreciated by an illustration: If all the men in a country were on anaverage 6 feet high, and there were some families which had been long and closely interbred, these would be almost dwarfs, their average height during ten generations being only 4 feet 8} inches. Ht IPOM@A PURPUREA. Cuap. If, Tt should be especially observed that the average dif- ference between the crossed and self-fertilised plants is not due to a few of the former having grown to an extraordinary height, or to a few of the self-fertilised being extremely short, but to all the crossed plants having surpassed their self-fertilised opponents, with the few following exceptions. The first occurred in the sixth generation, in which the plant named “ Hero” appeared ; two in the eighth generation, but the self- fertilised plants in this generation were in an anomalous condition, as they grew at first at an unusual rate and conquered for a time the opposed crossed plants; and two exceptions in the ninth generation, though one of these plants only equalled its crossed opponent. ‘Therefore, of the seventy-three crossed plants, sixty- eight grew to a greater height than the self-fertilised plants, to which they were opposed. In the right-hand column of figures, the difference in height between the crossed and self-fertilised plants in the successive generations is seen to fluctuate much, as might indeed have been expected from the small number of plants measured in each generation being insufficient to give a fair average. It should be remembered that the absolute height of the plants goes for nothing, as each pair was measured as soon as one of them had twined up to the summit of its rod. The great difference in the tenth genera- tion, viz., 100 to 54, no doubt was partly accidental, though, when these plants were weighed, the differ- ence was even greater, viz. 100 to 44. The smallest amount of difference occurred in the fourth and the eighth generations, and this was apparently due to both the crossed and self-fertilised plants having become unhealthy, which prevented the former attains ing their usual degree of superiority. This was an Car. Il. SUMMARY OF OBSERVATIONS. 55 unfortunate circumstance, but my experiments were not thus vitiated, as both lots of plants were exposed to the same conditions, whether favourable or un- favourable. There is reason to believe that the flowers of this Ipomeea, when growing out of doors, are habitually crossed by insects, so that the first seedlings which I raised from purchased seeds were probably the offspring of a cross. I infer that this is the case, firstly from humble-bees often visiting the flowers, and from the quantity of pollen left by them on the stigmas of such flowers ; and, secondly, from the plants raised from the same lot of seed varying greatly in the colour of their flowers, for as we shall hereafter see, this indicates much intercrossing.* It is, therefore, remarkable that the plants raised by me from flowers which were, in all probability, self-fertilised for the first time after many generations of crossing, should have been so markedly inferior in height to the intercrossed plants as they were, namely, as 76 to100. As the plants which were self-fertilised in each succeeding generation necessarily became much more closely interbred in the later than in the earlier generations, it might have been expected that the difference in height between them and the crossed plants would have gone on in- creasing ; but, so far is this from being the case, that the difference between the two sets of plants in the seventh, eighth, and ninth generations taken together is less than in the first and second generations together. When, however, we remember that the self-fertilised and crossed plants are all descended from the same * Verlot says (‘Sur la Produc- color, cannot be kept pure unless tion des Variétés” 1865, p. 66) grown at a distance from all other that certain varieties of a closely _ varieties, allied plant, the Convolvulus tri- 56 IPOM@A PURPUREA. Cuar. IL mother-plant, that many of the crossed plants in each generation were related, often closely related, and that all were exposed to the same conditions, which, as we shall hereafter find, is a very important circumstance, it is not at all surprising that the difference between them should have somewhat decreased in the later generations. It is, on the contrary, an astonishing fact, that the crossed plants should have been victorious, even to a slight degree, over the self-fertilised plants of the later generations. The much greater constitutional vigour of the crossed than of the self-fertilised plants, was proved on five occasions in various ways; namely, by exposing them, while young, to a low temperature or to a sudden change of temperature, or by growing them, under very unfavourable conditions, in competition with full-grown plants of other kinds. With respect to the productiveness of the crossed and self-fertilised plants of the successive generations, my observations unfortunately were not made on any uniform plan, partly from the want of time, and partly from not having at first intended to observe more than a single generation. A summary of the results is here given in a tabulated form, the fertility of the crossed plants being taken as 100. First Generation of crossed and self-fertilised Plants growing in competition with ene another.—Sixty-five capsules produced from flowers on five crossed plants fertilised by pollen from a distinct plant, and fifty-five capsules produced from flowers on five self-fertilised plants fertilised by their own pollen, contained seeds in the proportion of . - 100 to 93 Fifty-six spontaneously eel fertiliced Caeanlen on the above five crossed plants, and twenty-five sponta- neously self-fertilised capsules on the above five self- fertilised plants, yielded seeds in the proportion of . 100 to 99 Cup. IT. SUMMARY OF OBSERVATIONS. Combining the total number of capsules produced by these plants, and the average number of seeds in each, the above crossed and self-fertilised plants yielded seeds in the proportion of . Other plants of this first generation grown eae unfavourable conditions and spontaneously self-ferti- lised, yielded seeds in the proportion of . Third Generation of crossed and self-fertilised Plants, —Crossed capsules compared with self-fertilised cap- sules contained seeds in the ratio of An equal number of crossed and self- Sonialeaee plants, both spontaneously self-fertilised, iia capsules in the ratio of . And these capsules contained seeds i in the ratio of . Combining these data, the productiveness of the crossed to the self-fertilised plants, both spontane- ously self-fertilised, was as Fourth Generation of crossed and self Pees Blane —Capsules from flowers on the crossed plants ferti- lised by pollen from another plant, and capsules from flowers on the self-fertilised plants fertilised with their own pollen, contained seeds in the proportion of Fifth Generation of crossed and self-fertilised Plants. —The crossed plants produced spontaneously a vast number more pods (not actually counted) than the self-fertilised, and these contained seeds in the pro- portion of Ninth Gaeeaen of pone” Te self ~fer ee Plants —Fourteen crossed plants, spontaneously self-ferti- lised, and fourteen self-fertilised plants spontaneously self-fertilised, yielded capsules (the average number of seeds per capsule not having been ema in the proportion of . : “| Plants derived from a cross with a ee Stock com= pared with intercrossed Plants.—The offspring of inter- crossed plants of the ninth generation, crossed by a fresh stock, compared with plants of the same stock intercrossed during ten generations, both sets of plants left uncovered and naturally fertilised, produced capsules by weight as 57 100 to 64 100 to 45 100 to 94 100 to 38 100 to 94 100 to 35 100 to 94 100 to 89 100 to 26 100 to 51 We see in this table that the crossed plants are 58 IPOMG@A PURPUREA. Crap. I. always in some degree more productive than the self- fertilised plants, by whatever standard they are com- pared. The degree differs greatly; but this depends chiefly on whether ‘an average was taken of the seeds alone, or of the capsules alone, or of both combined. The relative superiority of the crossed plants is chiefly due to their producing a much greater number of cap- sules, and not to each capsule containing a larger average number of seeds. For instance, in the third generation the crossed and self-fertilised plants pro- duced capsules in the ratio of 100 to 38, whilst the seeds in the capsules on the crossed plants were to those on the self-fertilised plants only as 100 to 94. In the eighth generation the capsules on two self- fertilised plants (not included in the above table), grown in separate pots and thus not subjected to any competition, yielded the large average of 5'1 seeds. The smaller number of capsules produced by the self- fertilised plants may be in part, but not altogether, attributed to their lessened size or height; this being chiefly due to their lessened constitutional vigour, so that they were not able to compete with the crossed plants growing in the same pots. The seeds produced by the crossed flowers on the crossed plants were not always heavier than the self-fertilised seeds on the self-fertilised plants. The lighter seeds, whether pro- duced from crossed or self-fertilised flowers, generally germinated before the heavier seeds. I may add that the crossed plants, with very few exceptions, flowered before their self-fertilised opponents, as might have been expected from their greater height and vigour. The impaired fertility of the self-tertilised plants was shown in another way, namely, by their anthers being smaller than those in the flowers on the crossed plants. This was first observed in the seventh generation, but Cuar, IT, SUMMARY OF OBSERVATIONS. 59 may have occurred earlier. Several anthers from flowers on the crossed and self-fertilised plants of the eighth generation were compared under the microscope ; and those from the former were generally longer and plainly broader than the anthers of the self-fertilised plants. The quantity of pollen contained in one of the latter was, as far as could be judged by the eye, about half of that contained in one from a crossed plant. The impaired fertility of the self-fertilised plants of the eighth generation was also shown in another manner, which may often be observed in hybrids—namely, by the first-formed flowers being sterile. For instance, the fifteen. first flowers on a self-fertilised plant of one of the later generations were carefully fertilised with their own pollen, and eight of them dropped off; at the same time fifteen flowers on a crossed plant growing in the same pot were self-fertilised, and only one dropped off. On two other crossed plants of the same generation, several of the earliest flowers were observed to fertilise themselves and to produce capsules. In the plants of the ninth, and I believe of some previous generations, very many of the flowers, as already stated, were slightly monstrous; and this probably was connected with their lessened fertility. All the self-fertilised plants of the seventh genera- tion, and I believe of one or two previous generations, produced flowers of exactly the same tint, namely, of a rich dark purple. So did all the plants, without any exception, in the three succeeding generations of self- fertilised plants ; and very many were raised on account of other experiments in progress not here recorded. My attention was first called to this fact by my gardener remarking that there was no occasion to label the self-fertilised plants, as they could always be known by their colour. The flowers were as uniform in tint 60 IPOM@A PURPUREA. Crap. IL, as those of a wild species growing in a state of nature ; whether the same tint occurred, as is probable, in the earlier generations, neither my gardener nor self could recollect. The flowers on the plants which were first raised from purchased seed, as well as during the first few generations, varied much in the depth of the purple tint; many were more or less pink, and occa- sionally a white variety appeared. The crossed plants continued to the tenth generation to vary in the same manner as before, but to a much less degree, owing, probably, to their having become more or less closely inter-related. We must therefore attribute the extra- ordinary uniformity of colour in the flowers on the plants of the seventh and succeeding self-fertilised generations, to inheritance not having been interfered with by crosses during several preceding generations, in combination with the conditions of life having been very uniform. A plant appeared in the sixth self-fertilised genera- tion, named the Hero, which exceeded by a little in height its crossed antagonist, and which transmitted its powers of growth and increased self-fertility to its children and grandchildren. A cross between the children of Hero did not give to the grandchildren any advantage over the self-fertilised grandchildren raised from the self-fertilised children. And as far as my observations can be trusted, which were made on very unhealthy plants, the great-grandchildren raised from intercrossing the grandchildren had no advantage over the seedlings from the grandchildren the product of continued self-fertilisation; and what is far more remarkable, the great-grandchildren raised by crossing the grandchildren with a fresh stock, had no advantage over either the intercrossed or self-fertilised great- grandchildren. It thus appears that Hero and its Cuar. II. SUMMARY OF OBSERVATIONS. 61 descendants differed in constitution in an extraordinary manner from ordinary plants of the present species. Although the plants raised during ten successive generations from crosses between distinct yet inter- related plants almost invariably exceeded in height, constitutional vigour, and fertility their self-fertilised opponents, it has been proved that seedlings raised by intercrossing flowers on the same plant are by no means superior, on the-contrary are somewhat inferior in height and weight, to seedlings raised from flowers fertilised with their own pollen. This is a remarkable fact, which seems to indicate that self-fertilisation is in some manner more advantageous than crossing, unless the cross brings with it, as is generally the case, some decided and preponderant advantage ;,but to this subject I shall recur in a future chapter. The benefits which so generally follow from a cross between two plants apparently depend on the two differing somewhat in constitution or character. This is shown by the seedlings from the intercrossed plants of the ninth generation, when crossed with pollen from. a fresh stock, being as superior in height and almost as superior in fertility to the again intercrossed plants, as these latter were to seedlings from self-fertilised plants of the corresponding gene- ration. We thus learn the important fact that the mere act of crossing two distinct plants, which are in some degree inter-related and which have been long subjected to nearly the same conditions, does little good as compared with that from a cross between plants belonging to different stocks or families, and which have been subjected to somewhat different con- ditions. We may attribute the good derived from the crossing of the intercrossed plants during the ten successive generations to their still differing some- 62 IPOM@A PURPUREA. Crap. IL what in constitution or character, as was indeed proved by their flowers still differing somewhat in colour. But the several conclusions which may be deduced from the experiments on Ipomcea will be more fully considered in the final chapters, after all my other observations have been given. } Guar, LIT. MIMULUS LUTEUS. 63 CHAPTER III. ScROPHULARIACEZ, GESNERIACEZ, LABIATE, ETO. Mimulus luteus; height, vigour, and fertility of the crossed and selfs fertilised plants of the first four generations—Appearance of a new, tall, and highly self-fertile variety—Offspring from a cross between self-fertilised plants—Effects of a cross with a fresh stock —KHEffects of crossing flowers on the same plant—Summary on Mimulus lInteus—Digitalis purpurea, superiority of the crossed plants—Hffects of crossing flowers on the same plant—Caleceolaria —Linaria vulgaris— Verbascum thapsus— Vandellia nummulari- folia—Cleistogamic flowers—Gesneria pendulina—Salvia coccinea— Origanum vulgare, great increase of the crossed plants by stolons —Thunbergia alata, In the family of the Scrophulariacese I experimented on species in the six following genera: Mimulus, Digitalis, Calceolaria, Linaria, Verbascum, and Van- dellia. II. SCROPHULARIACEAL.—Mimuuus LutTevs. The piants which I raised from purchased seed varied greatly in the colour of their flowers, so that hardly two individuals were quite alike; the corolla being of all shades of yellow, with the most diversified blotches of purple, crimson, orange, and coppery brown. But these plants differed in no other respect.* The flowers are evidently well adapted for fertilisa- tion by the agency of iusects; and in the case of a closely allied species, M. roseus, I have watched bees entering the flowers, thus getting their backs well dusted with pollen; and when they entered another flower the pollen was licked off their backs by * T sent several specimens with The flowers with much red have variously coloured flowers to Kew, been named by horticulturists as and Dr. Hooker informs me that var. Youngiana, they all consisted of M luteus. 64 MIMULUS LUTEUS. Cuap. IIL, the two-lipped stigma, the lips of which are irritable and close like a forceps on the pollen-grains. If no pollen is enclosed between the lips, these open again after a time. Mr. Kitchener has ingeniously explained * the use of these movements, namely, to prevent the self-fertilisation of the flower. If a bee with no pollen on its back enters a flower it touches the stigma, which quickly closes, and when the bee retires dusted with pollen, it can leave none on the stigma of the same flower. But as soon as it enters any other flower, plenty of pollen is left on the stigma, which will be thus cross-fertilised. Never- theless, if insects are excluded, the flowers fertilise themselves perfectly and produce plenty of seed; but I did not ascertain whether this is effected by the stamens increasing in length with advancing age, or by the bending down of the pistil. The chief interest in my experiments on the present species, lies in the appearance in the fourth self-fertilised generation of a variety which bore large peculiarly-coloured flowers, and grew to a greater height than the other varieties; it likewise became more highly self-fertile, so that this variety resembles the plant named Hero, which appeared in the sixth self-fertilised generation of Ipomcea. Some flowers on one of the plants raised from the purchased seeds were fertilised with their own pollen; and others on the same plant were crossed with pollen from a distinct plant. 'The seeds from twelve capsules thus produced were placed in separate watch-glasses for comparison; and those from the six crossed capsules appeared to the eye hardly more numerous than those from the six self-fertilised capsules. But when the seeds were weighed, those from the crossed capsules amounted to 1°02 grain, whilst those from the self-fertilised capsules were only 81 grain; so that the former were either heavier or more numerous than the latter, in the ratio of 100 to 79. Crossed and self-fertilised Plants of the First Generation. Having ascertained, by leaving crossed and self-fertilised seed on damp sand, that they germinated simultaneously, both kinds were thickly sown on opposite sides of a broad and rather shallow pan; so that the two sets of seedlings, which came up at the same time, were subjected to the same unfavourable conditions. This was a bad method of treatment, but this species was one of the first on which I experimented. When the crossed seedlings * ©A Year’s Botany,’ 1874, p. 118, Cusp. III. CROSSED AND SELF-FERTILISED PLANTS. 65 Were on an average half an inch high, the self-fertilised ones were only a quarter of an inch high. When grown to their full height under the above unfavourable conditions, the four tallest crossed plants averaged 7°62, and the four tallest self-fertilised 5°87 inchesin height; oras100to77. Ten flowers on the crossed. plants were fully expanded before one on the self-fertilised plants. A few of these plants of both lots were transplanted. into a large pot with plenty of good earth, and the self-fertilised plants, not now being subjected to severe competition, grew during the following year as tall as the crossed plants; but from a case which follows it is doubtful whether they would have long continued equal. Some flowers on the crossed plants were crossed with pollen from another plant, and the capsules thus produced contained a rather greater weight of seed than those on the self-fertilised plants again self-fertilised. Crossed and self-fertilised Plants of the Second Generation.—Seeds from the foregoing plants, fertilised in the manner just stated, were sown on the opposite sides of a small pot (I.) and came up crowded. The four tallest crossed seedlings, at the time of flowering, averaged 8 inches in height, whilst the four tallest self-fertilised plants averaged only 4 inches. Crossed seeds were sown by themselves in a second small pot, and self- fertilised seeds were sown by themselves in a third small pot ; so that there was no competition whatever between these two lots. Nevertheless the crossed plants grew from 1 to 2 inches higher on an average than the self-fertilised. Both lots looked equally vigorous, but the crossed plants flowered earlier and more profusely than the self-fertilised. In Pot I., in which the two lots competed with each other, the crossed plants flowered first and produced a large number of capsules, whilst the self-fertilised produced only nineteen. The contents of twelve capsules from the crossed flowers on the crossed plants, and of twelve capsules from self-fertilised flowers on the self-fertilised plants, were placed in separate watch-glasses for comparison and the crossed seeds seemed more numerous by half than the self-fertilised. The plants on both sides of Pot I., after they had seeded, were cut down and transplanted into a large pot with plenty of good earth, and in the following spring, when they had grown to a height of between 5 and 6 inches, the two lots were equal, as occurred in a similar experiment in the last generation. But after some weeks the crossed plants exceeded the self- B 66 MIMULUS LUTEUS. Cuar. IL fertilised ones on the opposite side of the same pot, though not nearly to so great a degree as before, when they were subjected to very severe competition. Crossed and self-fertilised Plants of the Third Generation— Crossed seeds from the crossed plants, and self-fertilised seeds from the self-fertilised plants of the last generation, were sown thickly on opposite sides of a small pot, No. I. The two tallest plants on each side were measured after they had flowered, and the two crossed ones were 12 and 73 inches, and the two self- fertilised ones 8 and 53 inches in height; that is, in the ratio of 100 to 69. Twenty flowers on the crossed plants were again crossed and produced twenty capsules; ten of which contained 1°33 grain weight of seeds. Thirty flowers on the self-fertilised plants were again self-fertilised and produced twenty-six capsules; ten of the best of which (many being very poor) con- tained only °87 grain weight of seeds; that is, in the ratio of 100 to 65 by weight. The superiority of the crossed over the self-fertilised plants was proved in various ways. Self-fertilised seeds were sown on one side of a pot, and two days afterwards crossed seeds on the opposite side. The two lots of seedlings were equal until they were above half an inch high; but when fully grown the two tallest crossed plants attained a height of 123 and 8% inches, whilst the two:tallest self-fertilised plants were only 8 and 53 inches high. Ina third pot, crossed seeds were sown four days after the self-fertilised, and the seedlings from the latter had at first, as might have been expected, an advantage; but when the two lots were between 5 and 6 inches in height, they were equal, and ultimately the three tallest crossed plants were 11, 10, and 8 inches, whilst the three tallest self-fertilised were 12, 82, and 74 inches in height. So that there was not much difference between them, the crossed plants having an average advantage of only the third of an inch. The plants were cut down, and without being disturbed were transplanted into a larger pot. Thus the two lots started fair in the following spring, and now the crossed plants showed their inherent superiority, for the two tallest were 13 inches, whilst the two tallest self-fertilised plants were only 11 and 8} inches in height; or as 100 to 75. The two lots were allowed to fertilise themselves spontaneously : the crossed plants produced a large number of capsules, whilst the nelf-fertilised produced very few and poor ones. The seeds Cuar. III. CROSSED AND SELF-FERTILISED PLANTS. 67 from eight of the capsules on the crossed plants weighed ‘65 grain, whilst those from eight of the capsules on the self-fer- tilised plants weighed only -22 grain; or as 100 to 24. The crossed plants in the above three pots, as in almost all the previous experiments, flowered before the self-fertilised. This occurred even in the third pot in which the crossed seeds were sown four days after the self-fertilised seeds. Lastly, seeds of both lots were sown on opposite sides of a large pot in which a Fuchsia had long been growing, so that the earth was full of roots. Both lots grew miserably; but the crossed seedlings had an advantage at all times, and ultimately attained to a height of 34 inches, whilst the self-fertilised seed- lings never exceeded linch. The several foregoing experiments prove in a decisive manner the superiority in constitutional vigour of the crossed over the self-fertilised plants. In the three generations now described and taken together, the average height of the ten tallest crossed plants was 8°19 inches, and that of the ten tallest self-fertilised plants 5°29 inches (the. plants having been grown in small pots), or as 100 to 65. In the next or fourth self-fertilised generation, several plants of a new and tall variety appeared, which increased in the later self-fertilised generations, owing to its great self-fertility, to the complete exclusion of the original kinds. The samo variety also appeared amongst the crossed plants, but as it was not at first regarded with any particular attention, I know not how far it was used for raising the intercrossed plants; and in the later crossed generations it was rarely present. Owing to the appearance of this tall variety, the comparison of the crossed and self-fertilised plants of the fifth and succeeding generations was rendered unfair, as all the self-fertilised and only a few or none of the crossed plants consisted of it. Nevertheless, the results of the later experiments are in some respects well worth giving. Crossed and self-fertilised Plants of the Fourth Generation.—Seeds of the two kinds, produced in the usual way from the two sets of plants of the third generation, were sown on opposite sides of two pots (I. and II.); but the seedlings were not thinned enough and did not grow well. Many of the self-fertilised plants, especially in one of the pots, consisted of the new and tall variety above referred to, which bore large and almost white flowers marked with crimson blotches. I will call it the White variety. I believé that it first appeared amongst both the Fr 2 68 MIMULUS LUTEUS, Cuap. IIL. crossed and self-fertilised: plants of the last generation; but neither my gardener nor myself could remember any such variety in the seedlings raised from the purchased seed. It must there- fore have arisen either through ordinary variation, or, judging from its appearance amongst both the crossed and self-fertilised plants, more probably through reversion to a formerly existing variety. In Pot I. the tallest crossed plant was 8} inches, and the tallest self-fertilised 5 inches in height. In Pot IT. the tallest crossed plant was 63 inches, and the tallest self-fertilised plant, which consisted of the white variety, 7 inches in height; and this was the first instance in my experiments on Mimulus in which the tallest self-fertilised plant exceeded the tallest crossed. Nevertheless, the two tallest crossed plants taken together were to the two tallest self-fertilised plants in height as 100 to 80. As yet the crossed plants were superior to the self-fertilised in fertility; for twelve flowers on the crossed plants were crossed and yielded ten capsules, the seeds of which weighed 1:7] grain. Twenty flowers on the self-fertilised plants were self-fertilised, and produced fifteen capsules, all appearing poor; and the seeds from ten of them weighed only °68 grain, so that from an equal number of capsules the crossed seeds were to the self- fertilised in weight as 100 to 40. Cressed and self-fertilised Plants of the Fifth Generation —Seeds from both lots of the fourth generation, fertilised in the usual manner, were sown on opposite sides of three pots. When the seedlings flowered, most of the self-fertilised plants were found to consist of the tall white variety. Several of the crossed plants in Pot I. likewise belonged to this variety, as did a very few in Pots II. and III. The tallest crossed plant in Pot I. was 7 inches, and the tallest self-fertilised plant on the opposite side 8 inches ; in Pots II. and III. the tallest crossed were 43 and 53, and the tallest self-fertilised 7 and 63 inches in height; so that the average height of the tallest plants in the two lots was as 100 for the crossed to 126 for the self-fertilised; and thus we have a complete reversal of what occurred in the four previous gene- rations. Nevertheless, in all three pots the crossed plants retained their habit of flowering before the self-fertilised. The plants were unhealthy from being crowded and from the extreme heat of the season, and were in consequence more or less sterile; but the crossed plants were somewhat less sterile than the self-fertilised plants, Cuar. III. SROSSED AND SELF-FERTILISED PLANTS. 69 Crossed and self-fertilised Plants of the Sixth Generation.—Seeds from plants of the fifth generation crossed and self-fertilised in the usual manner were sown on opposite sides of several pots. On the self-fertilised side every single plant belonged to the tall white variety. On the crossed side some plants belonged to this variety, but the greater number approached in character to the old and shorter kinds with smaller yellowish flowers blotched with coppery brown. When the plants on both sides were from 2 to 3 inches in height they were equal, but when fully grown the self-fertilised were decidedly the tallest and finest plants, but, from want of time, they were not actually measured. In half the pots the first plant which flowered was a self-fertilised one, and in the other half a crossed one. And now another remark- able change was clearly perceived, namely, that the self-fertilised plants had become more self-fertile than the crossed. The pots were all put under a net to exclude insects, and the crossed plants produced spontaneously only fifty-five capsules, whilst the self-fertilised plants produced eighty-one capsules, or as 100 to 147. The seeds from nine capsules of both lots were placed in separate watch-glasses for comparison, and the self-fertilised appeared rather the more numerous. Besides these sponta- neously self-fertilised capsules, twenty flowers on the crossed plants again crossed yielded sixteen capsules; twenty-five flowers on the self-fertilised plants again self-fertilised yielded seventeen capsules, and this is a larger proportional number of capsules than was produced by the self-fertilised flowers on the self-fertilised plants in the previous generations. The contents of ten capsules of both these lots were compared in separate watch-glasses, and the seeds from the self-fertilised appeared decidedly more numerous than those from the crossed plants. Crossed and self-fertilised Plants of the Seventh Generation.— Crossed and self-fertilised seeds from the crossed and self-ferti- -lised plants of the sixth generation were sown in the usual manner on opposite sides of three pots, and the seedlings were well and equally thinned. Every one of the self-fertilised plants (and many were raised) in this, as well as in the eighth and ninth generations, belonged to the tall white variety. Their uniformity of character, in comparison with the seedlings first raised from the purchased seed, was quite remarkable. On the other hand, the crossed plants differed much in the tints of their flowers, but not, I think, to so great a degree as those first raised. I determined this time to measure the plants on both sides 70 MIMULUS LUTEUS. Cuar. Iii. carefully. The self-fertilised seedlings came up rather before the crossed, but both lots were for a time of equal height. When first measured, the average height of the six tallest crossed plants in the three pots was 7°02, and that of the six tallest self-fertilised plants 8°97 inches, or as 100 to 128. When fully grown the same plants were again measured, with the result shown in the following table :— TABLE XVIII. (Seventh Generation.) No. of Pot. Crossed Plants. | Self-fertilised Plants, Inches, Inches. ike 113 195 11% 18 II 128 | 183 113 148 A. Wah Hl 9 3 12§ 113 11 Total inches. 68°63 93°88 The average height of the six crossed is here 11°43, and that of the six self-fertilised 15°64, or as 100 to 187. As it was now evident that the tall white variety transmitted its characters faithfully, and as the self-fertilised plants consisted exclusively of this variety, it was manifest that they would always exceed in height the crossed plants which belonged chiefly to the original shorter varieties. This line of experiment was therefore discontinued, and I tried whether intercrossing two self-fertilised plants of the sixth generation, growing in dis- tinct pots, would give their offspring any advantage over the offspring of flowers on one of the same plants fertilised with their own pollen. These latter seedlings formed the seventh genera- tion of self-fertilised plants, like those in the right-hand column in Table XVIII. ; the crossed plants were the product of six previous self-fertilised generations with an intercross in the last generation. The seeds were allowed to germinate on sand, and were planted in pairs on opposite sides of four pots, all the remaining seeds being sown crowded on opposite sides of Pot V.in Table XIX.; the three tallest on each side in this latter pot being alone measured. All the plants were twice measured—the first time whilst young, and theaverage height of the crossed plants Cuar. Ill. CROSSED AND SELF-FERTILISED PLANTS. 71 to that of the self-fertilised was then as 100 to 122. When fully grown they were again measured, as in the following table :— TABLE XIX. Intercrossed Plants | s.1¢ fertilised Plants from Self-fertilised No. of Pot. A of the Seventh eS wt Generation. ? Inches. Inches, I 128 153 i 0 3 113 10 a fat 143 atid I. 103 113 vi 3 113 12} 83 7 143 Ill. 133 | 103 123 | 11° IV; 7h 148 83 7 73 8 iat = | Ve 83 102 Crowded. 9 93 83 92 Total in inches, 159°38 175°50 The average height of the sixteen intercrossed plants is here 9-96 inches, and that of the sixteen self-fertilised plants 10-96, or as 100 to110; so that the intercrossed plants, the progenitors of which had been self-fertilised for the six previous generations, and had been exposed during the whole time to remarkably uni- form conditions, were somewhat inferior in height to the plants of the seventh self-fertilised generation. But as we shall pre- sently see that a similar experiment made after two additional generations of self-fertilisation gave a different result, I know not how far to trust the present one. In three of the five pots in Table XIX. a self-fertilised plant flowered first, and in the other two a crossed plant. These self-fertilised plants were remarkably fertile, for twenty flowers fertilised with their own pollen produced no less than nineteen very fine capsules ! "2 MIMULUS LUTEUS, Citar. II. The effects of a Cross with a distinct Stock.—Some flowers on the self-fertilised plants in Pot IV. in Table XIX. were fertilised with their own pollen, and plants of the eighth self-fertilised generation were thus raised, merely to serve as parents in the fol- lowing experiment. Several flowers on these plants were allowed to fertilise themselves spontaneously (insects being of course excluded), and the plants raised from these seeds formed the ninth self-fertilised generation ; they consisted wholly of the tall white variety with crimson blotches. Other flowers on the same plants of the eighth self-fertilised generation were crossed with pollen taken from another plant of the same lot; so that the seedlings thus raised were the offspring of eight previous generations of self-fertilisation with an intercross in the last generation; these I will call the intercrossed plants. Lastly, other flowers on the same plants of the eighth self-fertilised generation were crossed with pollen taken from plants which had been raised from seed procured from a garden at Chelsea. The Chelsea plants bore yellow flowers blotched with red, but differed in no other respect. They had been grown out of doors, whilst mine had been cultivated in pots in the greenhouse for the last eight generations, and in a different kind of soil. The seedlings raised from this cross with a wholly different stock may be called the “ Chelsea-crossed.” The three lots of seeds thus obtained were allowed to germinate on bare sand; and whenever a seed in all three lots, or in only two, germinated at the same time, they were planted in pots superficially divided into three or two compartments. The remaining seeds, whether or not in a state of germination, were thickly sown in three divisions ina large pot, X.,in Table XX. When the plants had grown to their full height they were measured, as shown in the following table; but only the three tallest plants in each of the three divisions in Pot X. were measured. In this table the average height of the twenty-eight Chelsea- crossed plants is 21°62 inches; that of the twenty-seven inter- crossed plants 12°2; and that of the nineteen self-fertilised 10°44. But with respect to the latter it will be the fairest plan to strike out two dwarfed ones (only 4 inches in height), so as not to exaggerate the inferiority of the self-fertilised plants; and this will raise the average height of the seventeen remaining self-fertilised plants to 11°2 inches. Therefore the Chelsea-crossed are to the inter- crossed in height as 100 to 56; the Chelsea-crossed to the self- fertilised as 100 to 52; and the intercrossed to the self-fertilised Cuap. III. CROSS WITH A FRESH STOCK. 73 TABLE XX. Plants from Self- | Plants from an in- ee ian ee fertilised Plants of | tercross between Generation trom No. of Pot. the Eighth Genera-| the Plants of the Plants oth tion crossed by Highth Self-ferti- F 5 ie Eighth Self-ferti- Chelsea Plants. lised Generation, RIG EG | Inches. Inches, Inches, ik 303 14 94 283 138 103 : 134 10 Il. | 208 114 118 | 223 12 123 i a : 3 III. 238 122 8 243 11; 258 LY. 92 4 8} 133 11 Vs 44 11 13 z 134 VI. 186 1 q 16} 4 124 Wille 124 15 ‘ 243 122 : 204 113 ‘ 264 153 Vill. 173 133 56 228 143 oe 27 143 ae IX, 226 118 ne 6 17 aa 202 147 ate X. 18} 92 103 Crowded Plants. 16; 83 84 173 10 114 Total inches. 605°38 329°50 : 198° 50 74 MIMULUS LUTEUS. Cuar. IL as 100 to 92. We thus see how immensely superior in height the Chelsea-crossed are to the intercrossed and to the self- fertilised plants. They began to show their superiority when only one inch high. They were also, when fully grown, much more branched with larger leaves and somewhat larger flowers than the plants of the other two lots, so that if they had been weighed, the ratio would certainly have been much higher than that of 100 to 56 and 52. The intercrossed plants are here to the self-fertilised in height as 100 to 92; whereas in the analogous experiment given in Table XIX. the intercrossed plants from the self- fertilised plants of the sixth generation were inferior in height to the self-fertilised plants in the ratio of 100 to 110. I doubt whether this discordance in the results of the two experiments can be explained by the self-fertilised plants in the present case having been raised from spontaneously self- fertilised seeds, whereas in the former case they were raised from artificially self-fertilised seeds; nor by the present plants having been self-fertilised during two additional generations, though this is a more probable explanation. With respect to fertility, the twenty-eight Chelsea-crossed plants produced 272 capsules; the twenty-seven intercrossed. plants produced 24; and the seventeen self-fertilised plants 17 capsules. All the plants were left uncovered so as to be naturally fertilised, and empty capsules were rejected. Capsules. Therefore 20 Chelsea-crossed plants would have produced 194-29 “4 20 Intercrossed plants iy ‘A Garg of 20 Self-fertilised plants i. Hy 20°0C Grains. The seeds contained in 8 capsules from the Chelsea- neh crossed plants weighed : The seeds contained in 8 capsules from the Tniercrosted } 0°51 plants weighed é The seeds contained in 8 capsules from the Self- fertilised Lo-s3 plants weighed If we combine the number of capsules produced together with the average weight of contained seeds, we get the following extraordinary ratios: Weight of seed produced by the same number 100 to 4 of Chelsea-crossed and intercrossed plants . } = Cuap. IIT. FLOWERS ON SAME PLANT CROSSED. 79 Weight of seed produced by the same number as 100 to 3 of Chelsea-crossed and self-fertilised plants Weight of seeds produced by the same number } of intercrossed and self-fertilised plants eum ie It is also a remarkable fact that the Chelsea-crossed plants exceeded the two other lots in hardiness, as greatly as they did in height, luxuriance, and fertility. In the early autumn most of the pots were bedded out in the open ground; and this always injures plants which have been long kept in a warm greenhouse. All three lots consequently suffered greatly, but the Chelsea-~ crossed plants much less than the other two lots. On the 8rd of October the Chelsea-crossed plants began to flower again, and continued to do so for some time ; whilst not a single flower was produced by the plants of the other two lots, the stems of which were cut almost down to the ground and seemed half dead. Early in December there was a sharp frost, and the stems of Chelsea-crossed were now cut down; but on the 23rd of December they began to shoot up again from the roots, whilst all the plants of the other two lots were quite dead. Although several of the self-fertilised seeds, from which the plants in the right-hand column in Table XX. were raised, germi- nated (and were of course rejected) before any of those of the other two lots, yet in only one of the ten pots did a self- fertilised plant flower before the Chelsea-crossed or the inter- crossed plants growing in the same pots. The plants of these two latter lots flowered at the same time, though the Chelsea- crossed grew so much taller and more vigorously than the intercrossed. As already stated, the flowers of the plants originally raised from the Chelsea seeds were yellow; and it deserves notice that every one of the twenty-eight seedlings raised from the tall white variety fertilised, without being castrated, with pollen from the Chelsea plants, produced yellow flowers; and: this shows how prepotent this colour, which is the natural one of the species, is over the white colour. The Effects on the Offspring of intercrossing Flowers on the same Plant, instead of crossing distinct Individuals.—In all the foregoing experiments the crossed plants were the product of a cross between distinct plants. I now selected a very vigorous plant in Table XX., raised by fertilising a plant of the eighth self-fertilised generation with pollen from the Chelsea stock, 76 MIMULUS LUTEUS. Ouar. If. Several flowers on this plant were crossed with pollen from other flowers on the same plant, and several other flowers were fertilised with their own pollen. The seed thus produced was allowed to germinate on bare sand; and the seedlings were planted in the usual manner on the opposite sides of six pots. r of naturally fertilised capsules produced— By the English-crossed and self-fertilised plants, as 100 to 63 By the English-crossed and intercrossed plants, «as 100 to 81 By the intercrossed and self=fertilised plants, as 100 to 78 But to have ascertained the productiveness of the three lota of plants, it would have been necessary to know how many capsules were produced by the same number of plants. The a 116 ESCHSCHOLTZIA CALIFORNICA. Cuap. IV. three long rows, however, were not of quite equal lengths, and the plants were much crowded, so that it would have been ex- tremely difficult to have ascertained how many capsules were produced by them, even if I had been willing to undertake so laborious a task as to collect and count all the capsules. But this was feasible with the plants grown in pots in the green- house; and although these were much less fertile than those growing out of doors, their relative fertility appeared, after care- fully observing them, to be the same. The nineteen plants of the English-crossed stock in the pots produced altogether 240 capsules; the intercrossed plants (calculated as nineteen) pro- duced 137°22 capsules; and the nineteen self-fertilised plants, 152 capsules. Now, knowing the number of seeds contained in forty-five capsules of each lot, it is easy to calculate the relative numbers of seeds produced by an equal number of the plants of the three lots. Number of seeds produced by an equal number of naturally- fertilised plants. Seeds. Plants of English-crossed and _ self-fertilised parentage . - as 100 to 40 Plants of the Finglish-crossed and Gntererasced parentage . : - a8 100 to 45 Plants of the interecossed and self. feenieed parentage . ; , . ‘ ‘ - as 100 to 89 The superiority in productiveness of the intercrossed plants (that is, the product of a cross between the grandchildren of the plants which grew in Brazil) over the self-fertilised, small as it is, is wholly due to the larger average number of seeds contained in the capsules; for the intercrossed plants produced fewer cap- sules in the greenhouse than did the self-fertilised plants. The great superiority in productiveness of the English-crossed over the self-fertilised plants is shown by the larger number of gapsules produced, the larger average number of contained seeds, and the smaller number of empty capsules. As the English- crossed and intercrossed plants were the offspring of crosses in every previous generation (as must have been the case from the flowers being sterile with their own pollen), we may conclude that the great superiority in productiveness of the English-crossed over the intercrossed plants is due to the two parents of the former having been long subjected to different conditions, Onar. IV. RESEDA LUTEA, hy The English-crossed plants, though so superior in productive- ness, were, as we have seen, decidedly inferior in height and weight to the self-fertilised, and only equal to, or hardly superior to, the intercrossed plants. Therefore, the whole advantage of a cross with a distinct stock is here confined to productiveness, and I have met with no similar case. VIII. RESEDACEA.—REsEDA LUTEA, Seeds collected from wild plants growing in this neighbour- hood were sown in the kitchen-garden; and several of the seedlings thus raised were covered with a net. Of these, some were found (as will hereafter be more fully described) to be absolutely sterile when left to fertilise themselves spontaneously, although plenty of pollen fell on their stigmas; and they were equally sterile when artificially and repeatedly fertilised with their own pollen; whilst other plants produced a few spon- taneously self-fertilised capsules. The remaining plants were left uncovered, and as pollen was carried from plant to plant by the hive and humble-bees which incessantly visit the flowers, they produced an abundance of capsules. Of the necessity of pollen being carried from one plant to another, I had ample evidence in the case of this species end of 2. odorata; for those plants, which set no seeds or very few as long as they were protected from insects, became loaded with capsules immediately that they were uncovered. Seeds from the flowers spontaneously self-fertilised under tho net, and from flowers naturally crossed by the bees, were sown on opposite sides of five large pots. The seedlings were thinned as soon as they appeared above ground, so that an equal number were left on the two sides. After a time the pots were plunged into the open ground. The same number of plants of crossed and self-fertilised parentage were measured up to the summits of their flower-stems, with the result given in the following table (XXXY.). Those which did not produce flower-stems were not measured. The average height of the twenty-four crossed plants is here 17°17 inches, and that of the same number of self-fertilised plants 14°61; or as 100 to 85. Of the crossed plants all but five flowered, whilst several of the self-fertilised did not doso. The above pairs, whilst still in flower, but with some capsules already formed, were afterwards cut down and weighed. The crossed 4 } zt 118 -’ RESEDA LUTEA. Cur. IV, weighed 90°5 ounces; and an equal number of the self-fertilised only 19 ounces, or as 100 to 21; and this is an astonishing difference. TABLE XXXYV. Reseda lutea, in pots. No. of Pot. Crossed Plants. Self-fertilised Plants. Inches. Inches. I 21 12% 143 16 193 113 7 153 15} 134 I 20% 123 173 163 23% 16; 174 133 208 133 SAI MeN La Nes nA ST IIL. 16} 144 178 194 163 20% 10 7% 10 173 IV. 223 9 19 11; 187 11 164 } 16 193 163 V. 253 148 22 16 88 143 143 143 Total in inches. 412°25 350°88 Seeds of the same two lots were also sown in two adjoining rows in the open ground. ‘There were twenty crossed plants in the one row and thirty-two self-fertilised plants in the other row, so that the experiment was not quite fair; but not so un- fair as it at first appears, for the plants in the same row were not crowded so much as seriously to interfere with each other’s growth, and the ground was bare on the outside of both rows. s Cuar. IV. RESEDA ODORATA. 119 These plants were better nourished than those in the pcts and grew to a greater height. The eight tallest plants in each row were measured in the same manner as before, with the following result :— TABLE XXXVI. Reseda lutea, growing in the open ground. Crossed Plants. Self-fertilised Plants. Inches, Inches, 28 333 272 23 273 213 286 208 29% 21% 268 22 26% 213 303 213 224°75 185°13 The average height of the crossed plants, whilst in full flower, was here 28°09, and that of the self-fertilised 23°14 inches; or as 100 to 82. It is a singular fact that the tallest plant in the two rows, was one of the self-fertilised. The self-fertilised plants had smaller and paler green leaves than the crossed. All the plants in the two rows were afterwards cut down and weighed. The twenty crossed plants weighed 65 ounces, and twenty self-ferti- lised (by calculation from the actual weight of the thirty-two self- fertilised plants) weighed 26°25 ounces; or as100 to 40. There- fore the crossed plants did not exceed in weight the self-fertilised plants in nearly so great a degree as those growing in the pots, owing probably to the latter having been subjecied to more severe mutual competition. On the other hand, they exceeded the self-fertilised in height in a slightly greater degree. ; RESEDA ODORATA. Plants of the common mignonette were raised from purchased seed, and several of them were placed under separate nets. Of these some became loaded with spontaneously self-fertilised cap- sules; others produced a few, and others not a single one. It must not be supposed that these latter plants produced no seed 1203 ' ss RESEDA ODORATA. Citar. IV. because their stigmas did not receive any pollen, for they were repeatedly fertilised with pollen from the same plant with no effect ; but they were perfectly fertile with pollen from any other plant. Spontaneously self-fertilised seeds were saved from one of the highly self-fertile plants, and other seeds were collected from the plants growing outside the nets, which had been crossed by the bees. These seeds after germinating on sand were planted in pairs on the opposite sides of five pots. The plants were trained up sticks, and measured to the summits of their leafy stems—the flower-stems not beingincluded. We here have the result :— TABLE XXXVII. Reseda odorata (seedlings from a highly Self-fertile Plant). No. of Pot. Crossed Plants. Self-fertilised Plants. Inches. ; ies Te 207 22% 34¢ 283 268 233 328 304 II 343 283 343 303 118 23 333 304 Ill. 173 44 27 25 50} 263 302 255 IY. enn 228 28 254 323 15} 323 248 V. 21 118 253 193 26 104 Total in inches. 522°25 428°50 The average height of the nineteen crossed plants is here 27°48, and that of the nineteen self-fertilised 22°55 inches ; or as 100 to 82. All these plants were cut down in the early autumn Cuar. IV. RESEDA ODORAT'A. 121 and weighed: the crossed weighed 11°5 ounces, and the self- fertilised 7°75 ounces, or as 100 to 67. hese two lots having been left freely exposed to the visits of insects, did not present any difference to the eye in the number of seed-capsules which they produced. The remainder of the same two lots of seeds were sown in two adjoining rows in the open ground; so that the plants were ex- posed to only moderate competition. ‘The eight tallest on each side were measured, as shown in the following table :— TABLE XXVIII, Reseda odorata, growing in the open ground. Crossed Plants, Self-fertilised Plants. Inches, | Inches, ee ea 263 272 254 24 25 268 283 25 29% 262 255 273 26% 254 283 oehen 206° 13 216°75 | The average height of the eight crossed plants is 25°76, and ihat of the eight self-fertilised 27°09; or as 100 to 105. We here have the anomalous result of the self-fertilised plants being a little taller than the crossed; of which fact I can offer no explanaticn. It is of course possible, but not probable, that the labels may have been interchanged by accident. Another experiment was now tried: all the self-fertilised capsules, though very few in number, were gathered from one of - the semi-self-sterile plants under a net ; and as several flowers on this same plant had been fertilised with pollen from a distinct individual, crossed seeds were thus obtained. I expected that the seedlings from this semi-self-sterile plant would have profited in a higher degree from a cross, than did the seedlings from the fully self-fertile plants. But my anticipation was quite wrong, for they profited in a less degree. An analogous result followed in the case of Eschscholtzia, in which the offspring of the plants of Brazilian parentage (which were partially self-sterile) did not 123 RESEDA ODORATA. Cuar. IV profit more from a cross, than did the plants of the far more self-fertile English stock. The above two lots of crossed and self-fertilised seeds from the same plant of Reseda odorata, after germinating on sand, were planted on opposite sides of five pots, and measured as in the last case, with the following result :— TABLE XXXIX. Reseda odorata (seedlings from a semi-self-steyile Plant). No. of Pot. Crossed Plants. Self-fertilised Plants. mee cae Inches. 16 334 31 308 28 298 133 20 32 II 22 21§ 334 268 313 25% 324 304 III. 30} 172 325 298 314 248 325 343 Ife 19} 208 304 32 243 314 3508 368 Vv. 348 24% 37} 34 313 22) 33 374 Total im inches. 599°75 | 554°25 The average height of the twenty crossed plants is here 29:98, and that of the twenty self-fertilised 27°71 inches; or as 100 to 92. These plants were then cut down and weighed; and the crossed in this case exceeded the self-fertilised in weight by a mere trifle, viz.,in the ratio of 100 to 99. The two lots, left freely exposed to insects, seemed to be equally fertile. ‘The remainder of the seed was sown in two adjoining rows in Cuar. IY. VIOLA TRICOLOR.. 123 the open ground ; and the eight tallest plants in each row were measured, with the following result :— TABLE XL. Reseda odorata (seedlings from a semi-self-sterile Plant, planted in the open ground). —, Crossed Plants. Self-fertilised Planis, Inches Inches. 282 223 224 243 254 233 253 213 294 223 273 273 22% 273 263 192 rota an ¢207°38 188°38 The average height of the eight crossed plants is here 25°92, and that of the eight self-fertilised plants 23°54 inches; or as 100 to 90. IX. VIOLACEA.—VI0LA TRICOLOR. Whilst the flowers of the common cultivated heartsease are young, the anthers shed their pollen into a little semi-cylin- drical passage, formed by the basal portion of the lower petal, and surrounded by papille. ‘The pollen thus collected lies close beneath the stigma, but can seldom gain access into its cavity, except by the aid of insects, which pass their proboscides down this passage into the nectary.* Consequently when I covered up a large plant of a cultivated variety, it set only eighteen capsules, and most of these contained very few good seeds—several from only one to three; whereas an equally fine * The flowers of this plant Bennett, in ‘Nature,’ May 15, have been fully described by Sprengel, Hildebrand, Delpino, and H. Miller. The latter author sums up all the previous obser- vations in his ‘ Befruchtung der Blumen,’ and in ‘ Nature,’ Nov. 20, 1873, p. 44, See also Mr. A. W. 1873, p. 50; and some remarks by Mr. Kitchener, ibid. p. 143. The facts which follow on the effects of covering up a plant of V. tricolor have been quoted by Sir J. Lubbock in his ‘ British Wild Flowers, &e. p. 62, 124 VIOLA TRICOLOR. Cuap. IV. uncovered plant of the same variety, growing close by, produced 105 fine capsules. The few flowers which produce capsules when _ insects are excluded, are perhaps fertilised by the curling inwards of the petals as they wither, for by this means pollen-grains adhering to the papillee might be inserted into the cavity of tha stigma. But it is more probable that their fertilisation is effected. as Mr. Bennett suggests, by Thrips and certain minute beetles which haunt the flowers, and which cannot be excluded by any net. Humble-bees are the usual fertilisers ; but I have more than once seen flies (Lhingia rostrata) at work, with the under sides of their bodies, heads and legs dusted with pollen; and having marked the flowers which they visited, I found them after a few days fertilised.* It is curious for how long a time the flowers of the heartsease and of some other plants may be watched without an insect being seen to visit them. During the summer of 1841, I observed many times daily for more than a fortnight some large clumps of heartsease growing in my garden, before I saw a single humble-bee at work. During another summer I did the same, but at last saw some dark-coloured humble-bees visiting on three suc- cessive days almost every flower in several clumps; and almost all these flowers quickly withered and produced fine capsules. I presume that a certain state of the atmosphere is necessary for the secretion of nectar, and that as soon as this occurs the insects discover the fact by the odour emitted, and immediately frequent the flowers. As the flowers require the aid of insects for their complete * T should add that this fly apparently did not suck the nec- tar, but was attracted by the pa- pillz which surround the stigma. H. Miiller also saw a small bee, an Andrena, which could not reach the nectar, repeatedly inserting its proboscis beneath the stigma, where the papille are situated; so that these papilla must be in some way attractive to insects. A writer asserts (‘ Zoologist,’ vol. lii.-iv. p. 1225) that a moth (Plusia) frequently visits the flowers of the pansy. Hive-bees do not ordinarily visit them, but a case has been recorded (‘Gar- deners’ Chronicle,’ 1844, p, 374) of these bees doing so. H. Miiller has also seen the hive-bee at work, but only on the wild small- flowered form. He gives a list (‘ Nature,’ 1873, p. 45) of all the insects which he has seen visiting both the large and small-flowered forms. From his account, I sus- pect that the flowers of plants in a state of nature are visited more frequently by insects than those of the cultivated varieties. He has seen several butterflies suck- ing the flowers of wild plants, and this I have never observed in gardens, though I have watched the flowers during many years, 7 2 ee eeeEeEeEeEEOEeEeEeEeEeEeEeEeeeeEeEeEeEeEeEeeeeeeeeeeeeeeeeeeeeeee ee Cuap. IV, VIOLA TRICOLOR. 125 fertilisation, and as they are not visited by insects nearly so often as most other nectar-secreting flowers, we can understand the remarkable fact discovered by H. Miller and described by him in ‘ Nature,’ namely, that this species exists under two forms. One of these bears conspicuous flowers, which, as we have seen, require the aid of insects, and are adapted to be cross-fer- tilised by them; whilst the other form has much smaller and less conspicuously coloured flowers, which are constructed on a slightly different plan, favouring self-fertilisation, and are thus adapted to ensure the propagation of the species. The self- fertile form, however, is occasionally visited, and may be crossed by insects, though this is rather doubtful. In my first experiments on Viola tricolor I was unsuccessful in raising seedlings, and obtained only one full-grown crossed and self-fertilised plant. The former was 123 inches and the latter 8 inches in height. On the following year several flowers on a fresh plant were crossed with pollen from another plant, which was known to be a distinct seedling; and to this point it is im- portant to attend. Several other flowers on the same plant were fertilised with their own pollen. The average number of seeds in the ten crossed capsules was 18-7, and in the twelve self-fertilised capsules 12°83; or as 100 to 69. These seeds, after germinating on bare sand, were planted in pairs on the opposite sides of five pots. They were first measured when about a third of their full size, and the crossed plants then averaged 3°87 inches, and the self-fertilised only 2°00 inches in height; or as 100 to 52. They were kept in the greenhouse, and did not grow vigorously. Whilst in flower they were again measured to the summits of their stems (see Table XLI.), with the following result :— The average height of the fourteen crossed plants is here 5°58 inches, and that of the fourteen self-fertilised 2°37; or as 100 to 42. In four out of the five pots, a crossed plant flowered before any one of the self-fertilised ; as likewise occurred with the pair raised during the previous year. These plants without being disturbed were now turned out of their pots and planted in the open ground, so as to form five separate clumps. Early in the following summer (1869) they fiowered profusely, and being visited by humble-bees set many capsules, which were carefully collected from all the plants on both sides. The crossed plants produced 167 capsules, and the self-fertilised only 17; or as 100 to 10, So that the crossed plants were more than twice the p 126 - VIOLA TRICOLOR. Guar. IV. height of the self-fertilised, generally flowered first, and produced ten times as many naturally fertilised capsules. TABLE XLI. Viola tricolor. No. of Pot. Crossed Plants. Self-fertilised Plants. Inches. Inches, I, bho et bo y= AT, me oO oan Total in inches, By the early part of the summer of 1870 the crossed plants in all the five clumps had grown and spread so much more than the self-fertilised, that any comparison between them was superfluous. The crossed plants were covered with a sheet of bloom, whilst only a single self-fertilised plant, which was much finer than any of its brethren, flowered. The crossed and self- fertilised plants had now grown all matted together on the respective sides of the superficial partitions still separating them ; and in the clump which included the finest self-fertilised plant, I estimated that the surface covered by the crossed plants was about nine times as large as that covered by the self-fertilised plants. The extraordinary superiority of the crossed over the self-fertilised plants in all five clumps, was no doubt due to the crossed plants at first having had a decided advantage over the self-fertilised, and then robbing them more and more of their food during the succeeding seasors. But we should remember Cuar. IV. VIOLA TRICOLOR. 13 that the same result would follow in a state of nature even to a greater degree; for my plants grew in ground kept clear of weeds, so that the self-fertilised had to compete only with the crossed plants; whereas the whole surface of the ground is naturally covered with various kinds of plants, all of which have to struggle together for existence. The ensuing winter was very severe, and in the following spring (1871) the plants were again examined. All the self- fertilised were now dead, with the exception of a single branch on one plant, which bore on its summit a minute rosette of leaves about as large as a pea. On the other hand, all the crossed plants without exception were growing vigorously. So that the self-fertilised plants, besides their inferiority in other respects, were more tender. Another experiment was now tried for the sake of ascertaining how far the superiority of the crossed plants, or to speak more correctly, the inferiority of the self-fertilised plants, would be transmitted to their offspring. The one crossed and one self- fertilised plant, which were first raised, had been turned out of their pot and planted in the open ground. Both produced an abundance of very fine capsules, from which fact we may safely conclude that they had been cross-fertilised by insects. Seeds from both, after germinating on sand, were planted in pairs on the opposite sides of three pots. The naturally crossed seedlings TABLE XLII. Viola tricolor: seedlings from crossed and self-fertilised Plants, the parents of both sets having been left to be naturally fertilised, Naturally crossed Naturally crossed No. of Pot. Plants from artifi- Plants from self- cially crossed Plants. fertilised Plants, Inches. Inches, I 12) ; 118 83 IL, 133 96 10 11} Ill 144 114 138 113 | Total in inches. 75°38 | 61°88 128 ADONIS ZSTIVALIS. Cnar. IV. derived from the crossed plants flowered in all three pots before the naturally crossed seedlings derived from the self-fertilised plants. When both lots were in full flower, the two tallest plants on each side of each pot were measured, and the result is shown in the preceding table. 'The average height of the six tallest plants derived from tho crossed plants is 12°56 inches; and that of the six tallest plants derived from the self-fertilised plants is 10°31 inches; or as 100 to 82. We here see a considerable difference in height between the two sets, though very far from equalling that in the previous trials between the offspring from crossed and self- fertilised flowers. This difference must be attributed to the latter set of plants having inherited a weak constitution from their parents, the offspring of self-fertilised flowers ; notwith- standing that the parents themselves had been freely inter- crossed with other plants by the aid of insects. X. RANUNCULACEA.—ADONIS ZSTIVALIS. The results of my experiments on this plant are hardly worth giving, as I remark in my notes made at the time, “ seedlings, from some unknown cause, all miserably unhealthy.” Nor did they ever become healthy; yet I feel bound to give the present case, as it is opposed to the general results at which I have arrived. Fifteen flowers were crossed and all produced fruit, containing on an average 32°5 seeds; nineteen flowers were fertilised with their own pollen, and they likewise all yielded fruit, containing a rather larger average of 84°5 seeds; or as 100 to 106. Seedlings were raised from these seeds. In one of te pots all the self- fertilised plants died whilst quite young; ix the two others, the measurements were as follows : TABLE XLIII. Adonis xstivalis. No. of Pot. Crossed Plants. Self-fertilised Plants. Inches. Inches I. 14 134 13 4 13 3 Il. | | Total in inches. | Cuar. LY. DELPHINIUM CONSOLIDA. 129 The average height of the four crossed plants is 14°25, and that of the four self-fertilised plants 14°31; or as 100 to 100°4; so that they were in fact of equal height. According 2S 205 Dis tog orgy os Pee es fo} ss S2o8 a ® ae os OOO 34 S Bi Ea Wen Lat be 2s A i ae eration, in pots, crowded . } 2 /16°75) 2/ 9°75) ,, 4 58 Dianthus caryophyllus—3rd gen- ; k eration, in pots . | 8 | 28°39) 8 | 28-21) ,, ,, 99 Dianthus caryophyllus—offspring) from plants of the 3rd self-fer bal lised generation crossed by ater 15 |28-00| 10 | 26-55 95 crossed plants of 3rd generation, compared with plants of 4th self- fertilised generation. . . 242 SUMMARY OF MEASUREMENTS. Cuar. VIL. TABLE A—continued. U Uae] O.. be Bh Be ete one es Qa a S 23 ns C5) 2a| Se [63] os meu S OS) Os Isa) Sa og ‘el wd jas S| Ve oF mie = 2n ae 23/22 |g) 32 | 33° ma ~ qa n cae ne Ps NAMES OF PLANTS. —~ A | BR | Pe @ & SS oe on | Ay SASsa ae ea ee Ee 88| 23 | 55) 23 See 25, Se 2s a SO. A hae £2] 55 5So8 =} > i) > oD > R23 1 | Dianthus caryophyllus — number and average height of all the} 31 plants of Dianthus . : Hibiscusyainicanusienuciels scale i) 14 4B. | 9 yy tihgy) LOD | VG G2 oy Nae se yi 46°00 | sgt ba ato Sig SON mas ol sametS 2A.) nas ape 30°50 yen eee 82°35) lou, qaunae Pelargonium zonale Tropeolum minus . Be Limnanthes douglasii . . . . Lupinus luteus—2nd generation Lupinus pilosus — plants of two Penerabions, Peyiiel elite es *} Phaseolus multiflorus . . . Pisum sativum. . . . Sarothamnus scoparius — small seedlings. . — QO Po NY WDADNK ist) Oo QV ioe) 27°37] 26 | 25°18 las 100 to 92 89681), i ale D> POW ODRoOYNA to a Tea mals 2°91 Sarothamnus scoparius—the three survivors on each side after three 18°91 a Rares rada He es (ote) years’ growth . . . Ononis minutissima . . 2 OST 2 OR ease eee Clarkiaelegans. . . . . 4 |33°50|. 4|27°62) ,, 4, 82 Banfoniayaunes vei ilo) ye Mie te 8) 24°62) 8126730 | lee OF Passiflora gracilis . . : 2 |49°00} 2/51°00] ,, ,, 104 Apium petroselinum . . . .| ? {jaca ? {aun \ Hiway: (0 Scabiosa atro-purpurea, . . Zana br pecan fialisyeeeteg |e aa eee tayo) Lactuca sativa—plants of two gen- ‘ ; : ELALIONS Woven n iis rte th tte He Fro ideg EI SSCL Rives ic Specularia speculum . . . . | 4/19°28/ 4/18°93| ,, ,, 98 Lobelia ramosa—lIst generation .| 4 |22°25) 41|18°37] ,, ,, 82 Lobelia ramosa—2nd generation . | 3 |23°33]/ 3119-00] ,, ,, 81 Lobelia fulgens—Ist generation . | 2 |34°75) 2 |44°25] ,, ,, 127 Lobelia fulgens—2nd generation . | 23 |29°82] 2327-10] ,, ,, 91 Nemophila insignis—half-grown . | 12 |11°10] 12 | 5°45; ,, ,, 49 Nemophila insignis—the same fully é ; SLOW vel ivory tial arent: \ alee 19°90; 5, » 60 Borago officinalis . . . « 41120568 [0 4) 2018 bjigy) | aoe Nolana/prostratare some jieiiel ii OA soll) oO) 1s° 40h ease tle Petunia violacea—Ist generation . 5 |30°80} 5/26°00) ,, ,, 84 Petunia violacea—2nd generation. | 4 |40°50| 6 1|26°25| , , 65 Petunia violacea—3rd generation . | 8 |40°96] 8 |53°87] ,, ,, 131 Petunia violacea—4th generation . | 15 |46°79| 14 | 32:3 en eh) | Cuar. VII. / TaBLE A—continued. SUMMARY OF MEASUREMENTS. NAMES OF PLANTS. Petunia violacea—4th generation, from a distinct parent . . ‘| Petunia violacea—5th generation . Petunia violacea—5th generation, in open ground . Petunia violacea — Number and average height of all the plants in pots of Petunia Nicotiana tabacum—l1st generation Nicotiana tabacum—2nd generation Nicotiana tabacum—3rd generation Nicotiana tabacum—3rd generation but raised from a distinct plant \ Nicotiana tabacum—number and average height of all the ane of Nicotiana. . Cyclamen persicum. . Anagallis collina Primula sinensis — a ” dimor 2 species. Fagopyrum esculentum—a dimor-| phic species . a Beta vulgaris—in pots. Beta yulgaris—in open gr ound Canna warscewiczi—plants of three generations . . ‘} Zea mays—in pots, whilst young, measured to tips of leaves 2} Zea mays—when full grown, after the death of some, measured to tips of leaves. Zea mays—when full grown, after the death of some, measured to tips of flowers Zea mays—grown in open ‘ground, measured to tips ofleaves . } Zea mays—grown in open ground, measured to tips of tlowers . Phalaris canariensis—in pots . . Phalaris canariensis—in open ground Number of the Crossed Plants measured. 10 11 12 Average height of Crossed Plants in inches, faa Number of the Seif-ferti- 10 sti 12 lised Plants measured. for) Con to (J) lop) pe J 35° 33° Average height of Self- fertilised Plants in inches. oo 3 “70 “34 >] & ” ERLE 2 INE a RRC TRC Tt RT ght of the rossed to the Self-ferti- lised Plants, the forme7 taken as 100. Average hei C *87'as 100 to 60 °93 61 61 244 SUMMARY OF MEASUREMENTS, Crap. VII. TABLE B.—Relative Weights of Plants from Parents crossed with Pollen from distinct Plants of the same Stock, and Self-fertilised. Number | Number | Weight of the NAMES OF PLANTS. of Crossed) of Self- | Crossed Plants ' Plants. ee taken as 100. ants. Ipomea purpurea—plants of the 10th Cacia Gia he Wa 6 6 as 100 to 44 Vandellia nummular ifolia—Ist genera- a a 8 i niga eG ee MY a7 Brassica pleracea mole generation . : 9 9 Le beet Eschscholtzia califor pee of the 19 1 2nd generation . *} 9 Te eS ee ie ae generation, st own ic o4 24 eo RU al Reseda ]utea—I1st generation, gr own in 8 8 10 open ground . . . i oy hee Reseda odorata—Ist generation, de- scended from a highly self- ee 19 19 Hien. (OY plant, grown in pots. . Reseda odorata—Ist generation, de- ; scended from a semi-self-sterile plant, : 20 20 ye gigs ; grown in pots. 1 Dianthus car yophyllus—plants of the 8 8 49 3rd generation. . "| Bae Nata Petunia violacea—plants of the oi 29 a1 op generation, in pots. 7 29 asin: Petunia violacea—plants of the 5th 10 6 generation, in open ground mie ee ea ts nT TR TT Cuar. VIL SUMMARY OF MEASUREMENTS. 245 TABLE C.—Relative Heights, Weights, and Fertility of Plants from Parents crossed by a fresh Stock, and from Parents either self- fertilised or intercrossed with Plants of the same Stock. NAMES OF PLANI'S AND NATURE OF THE EXPERIMENTS. Weight. fertilised or Intercrossed Parents Cross with a Fresh Stock. of the same Stock. Number of the Plants from Self- with a Fresh Stock taken as of the Plants from the Cross 100. Number of the Plants from a Average Height in inches and Average Height in inches and Weight Height, Weight, and Fertility Jpomea purpurea — offspring of) plants intercrossed for nine iy a erations and then crossed by a fresh stock, compared with plants of the 10th intercrossed PCNETALIONT fray we isi he) lil | 19 | 84°03] 19 | 65:78 as 100 to 7& Ipomea purpurea —offspring o plants intercrossed for nine gen- erations and then crossed by a i fresh stock, compared with{| ° ; 1 plants of the 10th intercrossed generation, in fertility . . . Mimulusluteus—oftspring of plants self-fertilised for eight genera- tions and then crossed by a fresh}| 28 | 21°62] 19 |10°44| ,,_ ,, stock, compared with plants of the 9th self-fertilised generation qr Ls) Mimulusluteus—offspring of plants self-fertilised for eight genera- 4 | tions and then crossed by : a fresh stock, compared with plants of the 9th self-fertilised a in fertility . . Mimulusluteus—offspring of plants self-fertilised for eight genera- tions and then crossed by a fresh stock, compared with the off- spring of a plant self-fertilised;] 28 |21°62| 27 |12°20); , , 56 for eight generations, and then intercrossed with another self- fertilised plant of the same gen- Grabions Wowk ek ebn ise is so : 246 SUMMARY OF MEASUREMENTS. Crap. VIL TABLE O—continued. ere rc er EAMES OF PLANTS AND NATURE OF THE EXPERIMENTS, Cross with a Fresh Stock. Weight. Number of the Plants from Self- fertilised or Intercrossed Parents of the same Stock. Average Height in inches and Weight. Height, Weight, and Fertility of the Plants from the Cross with a Fresh Stock taken as 100. Number of the Plants from a Average Height in inches and a | | me | ee | Mimulus luteus—offspring of plants) self-fertilised for eight genera- tions and then crossed by a fresh stock, compared with the off- spring of a plant self-fertilised for eight generations, and then intercrossed with another self- fertilised plant of the same = eration, in fertility . . . . oie oe las LOO toy 4 plants self-fertilised for two gen- erations and then crossed by a fresh stock, compared | Brassica oleracea — offspring = | plants of the 3rd self-fertilised generation, by weight . Iberis umbellata—offspring Pie English variety crossed slightly ditferent Algerine veel 39 |17°34 riety, compared with the self-f| ° noe fertilised offspring of the oR WALLED Mireyil be)! ie Iberis Cena elise from English variety, crossed by slightly different Algerine variety, compared with the self-{| ** fertilised offspring of the English VaTieby. im tereiityy is) ley) 6 iis “1 wn Eschscholtzia californica—offspring\ of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the 2nd self-fertilised generation) 19 |50:30| ,, ,, 109 Eschscholtzia californica—offspring of a Brazilian stock crossed by an English stock, compared with H plants of the Brazilian stock of{| ** 79 ay ae » » 118 the 2nd self-fertilised generation, | | LRMWOC AOL Hania et tw lila siuale | Cuar. VII, § SUMMARY OF MEASUREMENTS. 247 Pa pregnant ee SL C—continued. plants’ of the Brazilian stock of the 2nd self-fertilised aa ation in fertility . . of a Brazilian stock crossed ce an English stock, compared with plants of the Brazilian stock of the 2nd intercrossed generation, tt ed Sateee EARN ees ce) 3 POR Se) ci ee Mlenlae Trieact lee aul 2 (a8 | 3 = Be 22/3 lef | 8 ESS a) £ 3 2's S r= abe Ae iho aa; —. | eeow < ae aes jae See ake NAMES OF PLANTS AND NATURE ae) 23 ge.| sm | 283 OF THE EXPERIMENTS. aa| = less] 2 | Psa sel a [ose| mw | See = EB ioe g ms ba g aD Po) Lod =i sos g2| & (|49s| & me Ss a6 2 lees) € | goes Zi 4 Bes Walla ee SEA Eschscholtzia californica—offspring) of a Brazilian stock crossed by an English stock, compared with as 100 to 40 Eschscholtzia californica—offspring AON Ao w SOS Las ISS eo of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the 2nd intercrossed generation, INE Welo Glare eie ese yews Eschscholtzia californica—ofispring ee oe ve 2° ” ” 100 of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the 2nd intercrossed generation, BU Terciliiyan , es S00 ta 21 plants of the 5th intercrossed generation, in weight . . . Petunia violacea — offspring plants self-fertilised for four generations and then crossed by a fresh stock, compared with?) 10 | 35-67] 10 |38:27] ,, ,, 104 plants of the 5th intercrossed generation, grownin open ground, HOWL MIG sala Mey Nena biota. Petunia violacea — offspring _ of) plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the 5th intercrossed generation, grown in opens ground vn in weight. . . Pe CAA VAS a led nc ey LY Petunia violacea — offspring plants self-fertilised for fou generations and then crossed by a fresh stock, compared with}! .. ae oe oe Hi Ae rato | plants of the 5th intercrossed generation, grown in open ground, micanllung |) Sy dle Nicotiana tabacum—offspring o | plants self-fertilised for three ( generations and then crossed by a slightly different variety, com- 26 | pared with plants of the 4th self-fertilised gereration, grown not much crowded in Beads hein gis a | 68-29) 26) 44°67)", ge Cuapr. VIL. SUMMARY OF MEASUREMENTS. TABLE C—continued. AMES OF PLAN'S AND NATURE | OF THE EXPERIMENTS. Cross with a Fresh Stock. Weight. fertilised or Intercrossed Parents of the same Stock. Number of the Plants from Self- Number of the Plants from a Average Height ininches and Nicotiana tabacum—oflspring of) plants self-fertilised for three generations and then crossed by|/| pared with plants of the 4th Average Height in inches and Weight of the Plants from the Cross with a Fresh Stock taken as Height, Weight, and Fertility 100. | a slightly different variety, sa] 12 | 31°53} 12 | 17°21) as 10) to 54 self-fertilised generation, grown much crowded in pots, in height) Nicotiana tabacum—offspring of) plants self-fertilised for three generations and then crossed by a slightly different variety, com->) .. a ee si6 pared with plants of the 3 self-fertilised generation, grown much crowded in pots, in weight Nicotiana tabacum—offspring o plants self-fertilised for three generations and then crossed by a slightly different variety, com->| 20 |48°74| 20 | 35°20 pared with plants of the 4th self-fertilised generation, grown in open ground, in height . Nicotiana tabacum—offspring | plants self-fertilised for three | generations and then crossed by a slightly different variety, com->| oo ae Ai ee self-fertilised generation, grown pared with plants of the ms in open ground, in weight . Anagallis collina—offspring from a) ied variety crossed by a blue variety, compared with the self->] 3 |27°62| 3 | 18:21 fertilised offspring of the red varioty ck sie ° 9 ” ” Ld 37 a | bo 63 §6 ' 252 SUMMARY OF MEASUREMENTS. Cuar. VIL TABLE C—continued. ‘ Ste lee ae Bes Bai s “es g Bas wo ~ &s| 3 Ea a Bon 2@| 2 Fg |S | #23 Qn 22) ee ee: | Seaeeee NAMES OF PLANTS AND NATURE)*&| 34 2.) 2m | 28a 3 a =£3/ =o Ba OF THE EXPERIMENTS. =a Se ang me Ee Ss jen) sso ra) Bee 32 7) 53a = Sos 26 3 32% ia P=E=| a5 i fas B wess o°| & (gas| 4 | sea iy 4 ' Anagallis collina—offspring from « red variety crossed by a blue variety, compared with the self->| .. fertilised offspring of the red variety, in fertility . : Primula veris—oftspring from long- styled plants of the 3rd illegiti- mate generation, crossed by a fresh stock, compared with plants of the 4th illegitimate as 100 to € and self-fertilised generation .J Primula veris—oftspring from long-) styled plants of the 3rd illegiti- mate generation, crossed by a fresh stock, compared with plants of the 4th illegitimate and self-fertilised argue in fertility 2 2. Primula veris—offspring fra om es styled plants of the 3rd illegiti- mate generation, crossed by a fresh stock, compared a plants of the 4th illegitimate and self-fertilised generation, | fertility in following year 3 | 8 Primula veris (equal-styled, maar) Spal ela paneaaihr ge al plants self-fertilised for two| generations and then crossed by a> different variety, compared with plants of the 3rd self-fertilised ORRIN (5) gy Ge Primula veris (equal-styled, red-) flowered variety)—offspring from plants self-fertilised for two generations and then crossed bya>| .. different variety, comparea with plants of the 3rd self-fertilised generation, in fertility . . Cap. VIL TABLE C. 253 In these three tables the measurements of fifty-seven species, belonging to fifty-two genera and to thirty great natural families, are given. The species are natives of various parts of the world. The number of crossed plants, including those derived from a cross between plants of the same stock and of two different stocks, amounts to 1,101; and the number of self-fer- tilised plants (including a few in Table C derived from a cross between plants of the same old stock) is 1,076. Their growth was observed from the germi- nation of the seeds to maturity; and most of them were measured twice and some thrice. The various precautions taken to prevent either lot being unduly favoured, have been described in the introductory chapter. Bearing all these circumstances in mind, it may be admitted that we have a fair basis for judging of the comparative effects of cross-fertilisation and of self-fertilisation on the growth of the offspring. It will be the most convenient plan first to consider the results given in Table C, as an opportunity will thus be afforded of incidentally discussing some important points. If the reader will look down the right-hand column of this table, he will see at a glance what an extraordinary advantage in height, weight, and fer- tility the plants derived from a cross with a fresh stock or with another sub-variety have over the self-fertilised plants, as well as over the intercrossed plants of the same old stock. There are only two exceptions to this rule, and these are hardly real ones. In the case of Eschscholtzia, the advantage is confined to fertility. In that of Petunia, though the plants derived from a eross with a fresh stock had an immense superiority in height, weight, and fertility over the self-fertilised plants, they were conquered by the intercrossed plants of the same old stock in height and weight, but not 254 SUMMARY OF MEASUREMENTS. Onap. VIL in fertility. It. has, however, been shown that the superiority of these intercrossed plants in height and weight was in all probability not real; for if the two sets had been allowed to grow for another month, it is almost certain that those from a cross with the fresh stock would have been victorious in every way over the intercrossed plants. Before we consider in detail the several cases given in Table C, some preliminary remarks must be made. There is the clearest evidence, as we shall presently see, that the advantage of a cross depends wholly on the plants differing somewhat in constitution ; and that the disadvantages of self-fertilisation depend on the two parents, which are combined in the same hermaphrodite flower, having a closely similar constitution. A certain amount of differentiation in the sexual elements seems indispensable for the full fertility of the parents, and for the full vigour of the offspring. All the individuals of the same species, even those produced in a state of nature, differ somewhat, though often very slightly, from one another in external characters and.probably in constitution. This obviously holds good between the varieties of the same species, as far as external characters are concerned; and much evidence could be advanced with respect to their generally differmg somewhat in constitution. ‘There can hardly be a doubt that the differences of all kinds between the individuals and varieties of the same species depend largely, and as I believe exclusively, on their progenitors having been subjected to different conditions ; though the conditions to which the individuals of the same species are ex- posed in a state of nature often falsely appear to us the same. or instance, the individuals growing together are necessarily exposed to the same climate, and they seem to us at first sight to be subjected to identically Cuap. VIL. TABLE C. 255 the same conditions; but this can hardly be the case, except under the unusual contingency of each individual being surrounded by other kinds of plants in exactly the same proportional numbers. For the surround- ing plants absorb different amounts of various sub- stances from the soil, and thus greatly affect the nourishment and even the life of the individuals of any particular species. These will also be shaded and otherwise affected by the nature of the surrounding plants. Moreover, seeds often lie dormant in the ground, and those which germinate during any one year will often have been matured during very different seasons. Seeds are widely dispersed by various means, and some will occasionally be brought from distant stations, where their parents have grown under some- what different conditions, and the plants produced from such seeds will intercross with the old residents, thus mingling their constitutional peculiarities in all sorts of proportions. Plants when first subjected to culture, even in their native country, cannot fail to be exposed to greatly changed conditions of life, more especially from growing in cleared ground, and from not having to compete with many or any surrounding plants. They are thus enabled to absorb whatever they require which the soil may contain. Fresh seeds are often brought from distant gardens, where the parent- plants have been subjected to different conditions. Cultivated plants like those in a state of nature frequently intercross, and will thus mingle their constitutional peculiarities. On the other hand, as long as the individuals of any species are culti- vated in the same garden, they will apparently be subjected to more uniform conditions than plants in a state of nature, as the individuals have not to compete 256 SUMMARY OF MEASUREMENTS. Cuap. VIL with various surrounding species. The seeds sown at the same time ina garden have generally been matured during the same season and in the same place; and in this respect they differ much from the seeds sown by the hand of nature. Some exotic plants are not frequented by insects in their new home, and there- fore are not intercrossed; and this appears to be a highly important factor in the individuals acquiring uniformity of constitution. In my experiments the greatest care was taken that in each generation all the crossed and self-fertilised plants should be subjected to the same conditions. Not that the conditions were absolutely the same, for the more vigorous individuals will have robbed the weaker ones of nutriment, and likewise of water when the soil in the pots was becoming dry; and both lots at one end of the pot will have received a little more light than those at the other end. In the successive generations, the plants were subjected to somewhat different conditions, for the seasons necessarily varied, and they were sometimes raised at different periods of the year. But as they were all kept under glass, they were exposed to far less abrupt and great changes of temperature and moisture than are plants growing out of doors. With respect to the intercrossed plants, theit first parents, which were not related, would almost certainly have differed somewhat in constitution ; and such constitutional peculiarities would be variously mingled in each succeeding intercrossed generation, being sometimes augmented, but more commonly neutralised in a greater or less degree, and sometimes revived through reversion; just as we know to be the case with the external characters of crossed species and varieties. With the plants which were self-fertilised | during the successive generations, this latter important Crap. VII. TABLE C. yin ay source of some diversity of constitution will have been wholly eliminated ; and the sexual elements produced by the same flower must have been developed under as nearly the same conditions as it is possible to conceive. In Table C the crossed plants are the offspring of a cross with a fresh stock, or with a distinct variety; and they were put into competition either with self-fertilised plants, or with intercrossed plants of the same old stock. By the term fresh stock I mean a non-related plant, the progenitors of which have been raised during some generations in another garden, and have consequently been exposed to somewhat different conditions. In the case of Nicotiana, Iberis, the red variety of Primula, the common Pea, and perhaps Anagallis, the plants which were crossed may be ranked as distinct varieties or sub-varieties of the same species; but with Ipomea, Mimulus, Dianthus, and Petunia, the plants which were crossed differed exclusively in the tint of their flowers; and as a large proportion of the plants raised from the same lot of purchased seeds thus varied, the differences may be estimated as merely individual. Having made these preliminary remarks, we will now consider in detail the several cases given in Table C, and they are well worthy of full consideration. (1.) Ipomeea purpurea.—Plants growing in the same pots, and subjected in each generation to the same conditions, were intercrossed for nine consecutive generations. These intercrossed plants thus became in the later generations more or less closely inter-related. Flowers on the plants of the ninth intercrossed genera- tion were fertilised with pollen taken from a fresh stock, and seedlings thus raised. Other flowers on the same intercrossed plants were fertilised with pollen from another intercrossed plant, producing seedlings of the tenth intercrossed generation. These two sets of g 258 SUMMARY OF MEASUREMENTS. Crap, VIL seedlings were grown in competition with one another, and differed greatly in height and fertility. For the offspring from the cross with a fresh stock exceeded in height the intercrossed plants in the ratio of 100 to 78 ; and this is nearly the same excess which the inter- crossed had over the self-fertilised plants in all ten generations taken together, namely, as 100 to 77. The plants raised from the cross with a fresh stock were also greatly superior in fertility to the intercrossed, namely, in the ratio of 100 to 51, as judged by the relative weight of the seed-capsules produced by an equal number of plants of the two sets, both having been left to be naturally fertilised. It should be especially observed that none of the plants of either lot were the product of self-fertilisation. On the contrary, the intercrossed plants had certainly been crossed for the last ten generations and probably during all previous generations, as we may infer from the structure of the flowers and from the frequency of the visits of humble- bees. And so it will have been with the parent-plants of the fresh stock. The whole great difference in height and fertility between the two lots must be attributed to the one being the product of a cross with pollen from a fresh stock, and the other of a cross between plants of the same old stock. This species offers another interesting case. In the five first generations in which intercrossed and self- fertilised plants were put into competition with one another, every single intercrossed plant beat its self- fertilised antagonist, except in one instance, in which they were equal in height. But in the sixth gene- ration a plant appeared, named by me the Hero, re- markable for its tallness and increased self-fertility, and which transmitted its characters to the next three generations. The children of Hero were again self- Cuap. VIL. TABLE C. 259 fertilised, forming the eighth self-fertilised generation, and were likewise intercrossed one with another; but this cross between plants which had been subjected to the same conditions and had been self-fertilised during the seven previous generations, did not effect the least good ; for the intercrossed grandchildren were actually shorter than the self-fertilised grandchildren, in the ratio of 100 to 107. We here see that the mere act of crossing two distinct plants does not by itself benefit the offspring. This case is almost the converse of that in the last paragraph, in which the offspring profited so greatly by a cross with a fresh stock. A similar trial was made with the descendants of Hero in the following generation, and with the same result. But the trial cannot be fully trusted, owing to the extremely unhealthy condition of the plants. Subject to this same serious cause of doubt, even a cross with a fresh stock did not benefit the great- grandchildren of Hero; and if this were really the case, it is the greatest anomaly observed by me in all my experiments. (2.) Mimulus luteus—During the three first genera- tions the intercrossed plants taken together exceeded in height the self-fertilised taken together, in the ratio of 100 to 65, and in fertility in a still higher degree. In the fourth generation a new variety, which grew taller and had whiter and larger flowers than the old varieties, began to prevail, especially amongst the self- fertilised plants. This variety transmitted its characters with remarkable fidelity, so that all the plants in the later self-fertilised generations belonged to it. These consequently exceeded the intercrossed plants consider- ably in height. Thus in the seventh generation the intercrossed plants were to the self-fertilised in height as 100 to 1387. It is a more remarkable fact that the 82 260 SUMMARY OF MEASUREMENTS. Cuar. VIL self-fertilised plants of the sixth generation had become much more fertile than the intercrossed plants, judging by the number of capsules spontaneously produced, in the ratio of 147 to 100. This variety, which as we have seen appeared amongst the plants of the fourth self-fertilised generation, resembles in almost all its constitutional peculiarities the variety called Hero, which appeared in the sixth self-fertilised generation of Ipomeea. No other such case, with the partial ex- ception of that of Nicotiana, occurred in my experi- ments, carried on during eleven years. Two plants of this variety of Mimulus, belonging to the sixth self-fertilised generation, and growing in separate pots, were intercrossed ; and some flowers on the same plants were again self-fertilised. From the seeds thus obtained, plants derived from a cross betwee the self-fertilised plants, and others of the seventh self-fertilised generation, were raised. But this cross did not do the least good, the intercrossed plants being inferior in height to the self-fertilised, in the ratio of 100 to 110. This case is exactly parallel with that given under Ipomeea, of the grandchildrer. of Hero, and apparently of its great-grandchildren ; for the seedlings raised by intercrossing these plants were not in any way superior to those of the cor- responding generation raised from the self-fertilised flowers. ‘Therefore in these several cases the crossing of plants, which had been self-fertilised for several generations and which had been cultivated all the time under as nearly as possible the same conditions, was not in the least beneficial. Another experiment was now tried. Firstly, plants of the eighth self-fertilised generation were again self-fertilised, producing plants of the ninth self- rertilised generation. Sezondly, two of the plants of the Cuap. VIL TABLE C, 261 eighth self-fertilised generation were intercrossed one with another, as in the experiment above referred to; but this was now effected on plants which had been subjected to two additional generations of self-fer- tilisation. ‘Thirdly, the same plants of the eighth self- fertilised generation were crossed with pollen from plants of a fresh stock brought from a distant garden. Numerous plants were raised from these three sets of seeds, and grown in competition with one another. The plants derived from a cross between the self-fer- tilised plants exceeded in height by a little the self- fertilised, viz.,as 100 to 92; and in fertility in a greater degree, viz. as 100 to 73. Ido not know whether this difference in the result, compared with that in the previous case, can be accounted for by the increased deterioration of the self-fertilised plants from two additional generations of self-fertilisation, and the consequent advantage of any cross whatever, although merely between the self-fertilised plants. But however this may be, the effects of crossing the self-fertilised plants of the eighth generation with a fresh stock were extremely striking ; for the seedlings thus raised were to the self-fertilised of the ninth generation as 100 to 52 in height, and as 100 to 3 in fertility! ‘They were also to the intercrossed plants (derived from crossing two of the self-fertilised plants of the eighth generation) in height as 100 to 56, and in fertility as 100 to 4. Better evidence could hardly be desired of the potent influence of a cross with a fresh stock on plants which had been self-fertilised for eight generations, and had been cultivated all the time under nearly uniform conditions, in comparison with plants self-fertilised for nine generations continuously, or then once inter- crossed, namely in the last generation. (3.) Brassica oleraceaa—Some flowers on cabbage ; / 262 SUMMARY OF MEASUREMENTS. Cuap. VIL. plants of the second self-fertilised generation were crossed with pollen from a plant of the same variety brought from a distant garden, and other flowers were again self-fertilised. Plants derived from a cross with a fresh stock and plants of the third self-fertilised generation were thus raised. The former were to the self-fertilised in weight as 100 to 22; and this enormous difference must be attributed in part to the beneficial effects of a cross with a fresh stock, and in part to the deteriorating effects of self-fertilisa- tion continued during three generations. (4.) Iberis wmbellata.— Seedlings from a crimson English variety crossed by a pale-coloured variety which had been grown for some generations in Algiers, were to the self-fertilised seedlings from the crimson variety in height as 100 to 89, and as 100 to 75 in fertility. Jam surprised that this cross with another variety did not produce a still more strongly marked beneficial effect ; for some intercrossed plants of the crimson English variety, put into competition with plants of the same variety self-fertilised during three generations, were in height as 100 to 86, and in fertility as 100 to 75. The slightly greater difference in height in this latter case, may possibly be attributed to the deteriorating effects of self-fertilisation carried on for two additional generations. (5.) Eschscholtzia californica.—This plant offers an almost unique case, inasmuch as the good effects of a cross or the evil effects of self-fertilisation are confined to the reproductive system. Intercrossed and self- fertilised plants of the English stock did not differ in height (nor in weight, as far as was ascertained,) in _ any constant manner ; the self-fertilised plants usually having the advantage. So it was with the offspring of plants of the Brazilian stock, tried in the same Cuar. VII. TABLE C. 263 manner. The parent-plants, however, of the English stock produced many more seeds when fertilised with pollen from another plant than when self-fertilised ; and in Brazil the parent-plants were absolutely sterile unless they were fertilised with pollen from another plant. Intercrossed seedlings, raised in England from the Brazilian stock, compared with self-fertilised seed- lings of the corresponding second generation, yielded seeds in number as 100 to 89; both lots of plants being left freely exposed to the visits of insects. If we now turn to the effects of crossing plants of the Brazilian stock with pollen from the English stock,—so that plants which had been long exposed to very different conditions were intercrossed,—we find that the off- spring were, as before, inferior in height and weight to the plants of the Brazilian stock after two genera- tions of self-fertilisation, but were superior to them in the most marked manner in the number of seeds produced, namely, as 100 to 40; both lots of plants being left freely exposed to the visits of insects. In the case of Ipomcea, we have seen that the plants derived from a cross with a fresh stock were superior in height as 100 to 78, and in fertility as 100 to 51, to the plants of the old stock, although these had been intercrossed during the last ten generations, With Eschscholtzia we have a nearly parallel case, but only as far as fertility is concerned, for the plants derived from a cross with a fresh stock were superior in fertility in the ratio of 100 to 45 to the Brazilian plants, which had been artificially intercrossed in England for the two last generations, and which must have been naturally intercrossed by insects during all previous generations in Brazil, where otherwise they are quite sterile. (6.) Dianthus caryophyllus.—Plants _ self-fertilised 264 SUMMARY OF MEASUREMENTS. Car. VIL. for three generations were crossed with pollen from a fresh stock, and their offspring were grown in compe- tition with plants of the fourth self-fertilised genera- tion. ‘The crossed plants thus obtained were to the self-fertilised in height as 100 to 81, and in fertility (both lots being left to be naturally fertilised by insects) as 100 to 33. These same crossed plants were also to the off- spring from the plants of the third self-fertilised gene- ration crossed by the intercrossed plants of the cor- responding generation, in height as 100 to 85, and in fertility as 100 to 40. We thus see what a great advantage the offspring from a cross with a fresh stock had, not only over the self-fertilised plants of the fourth generation, but over the offspring from the self-fertilised plants of the third generation, when crossed by the intercrossed plants of the old stock. (7.) Pisum sativum.—tIt has been shown under the head of this species, that the several varieties in this country almost invariably fertilise themselves, owing to insects rarely visiting the flowers; and as the plants have been long cultivated under nearly similar conditions, we can understand why a cross between two individuals of the same variety does not do the least good to the offspring either in height or fertility. This case is almost exactly parallel with that of Mimulus, or that of the Ipomoea named Hero; for in these two instances, crossing plants which had been self-fertilised for seven generations did not at all benefit the offspring. On the other hand, a cross -between two varieties of the pea causes a marked superiority in the growth and vigour of the offspring, over the self-fertilised plants of the same varieties, as shown by two excellent observers, From my own Cuar. VIL. TABLE C. 265 observations (not made with great care) the offspring from crossed varieties were to self-fertilised plants in height, in one case as 100 to about 75, and in a second case as 100 to 60. (8.) Lathyrus odoratus——The sweet-pea is in tho same state in regard to self-fertilisation as the common pea; and we have seen that seedlings from a cross between two varieties, which differed in no respect except in the colour of their flowers, were to the self- fertilised seedlings from the same mother-plant in height as 100 to 80; and in the second generation as 100 to 88. Unfortunately I did not ascertain whether crossing two plants of the same variety failed to pro- duce any beneficial effect, but I venture to predict such would be the result. (9.) Petunia violacea—The intercrossed plants of the same stock in four out of the five successive gene- rations plainly exceeded in height the self-fertilised plants. The latter in the fourth generation were crossed by a fresh stock, and the seedlings thus obtained were put into competition with the self-fertilised plants of the fifth generation. The crossed plants exceeded the self-fertilised in height in the ratio of 100 to 66, and in weight as 100 to 28; but this difference, though so great, is not much greater than that between the intercrossed plants of the same stock in comparison with the self-fertilised plants of the corresponding generation. ‘This case, therefore, seems at first sight opposed to the rule that a cross with a fresh stock is much more beneficial than a cross between individuals of the same stock. But as with Eschscholtzia, the reproductive system was here chiefly benefited; for the plants raised from the cross with the fresh stock were to the self-fertilised plants in fertility, both lots being naturally fertilised, as 100 to 46, whereas the 2665 SUMMARY OF MEASUREMENTS. Cuar. VIL intercrossed plants of the same stock were to the self- fertilised plants of the corresponding fifth generation in fertility only as 100 to 86. Although at the time of measurement the plants raised from the cross with the fresh stock did not exceed in height or weight the intercrossed plants of the old stock (owing to the growth of the former not having been completed, as explained under the head of this species), yet they exceeded the intercrossed plants in fertility in the ratio of 100 to 54. This fact is interesting, as it shows that plants self-fertilised for four generations and then crossed by a fresh stock, yielded seedlings which were nearly twice as fertile as those from plants of the same stock which had been intercrossed for the five previous generations. We here see, as with Eschscholtzia and Dianthus, that the mere act of crossing, independently of the state of the crossed plants, has little efficacy in giving increased fertility to the offspring. The same conclusion holds good, as we have already seen, in the analogous cases of Tpomcea, Mimulus, and Dianthus, with respect to height. (10.) Nicotiana tabacum.—My plants were remark- ably self-fertile, and the capsules from the self-fertilised flowers apparently yielded more seeds than those which were cross-fertilised. No insects were seen to visit the flowers in the hothouse, and I suspect that the stock on which I experimented had been raised under glass, and had been self-fertilised during several previous generations ; if so, we can understand why, in the course of three generations, the crossed seedlings of the same stock did not uniformly exceed in height the self-ferti- lised seedlings. But the case is complicated by indi- vidual plants having different constitutions, so that some of the crossed and self-fertilised seedlings raised at the same time from the same parents behaved differently. Cuar. VIL. TABLE C. 267 However this may be, plants raised from self-fertilised plants of the third generation crossed by a slightly different sub-variety, exceeded greatly in height and weight the self-fertilised plants of the fourth genera- tion; and the trial was made on a large scale. They exceeded them in height when grown in pots, and not. much crowded, in the ratio of 100 to 66; and when much crowded, as 100 to 54. These crossed plants, when thus subjected to severe competition, also exceeded the self-fertilised in weight in the ratio of 100 to 37. So it was, but in a less degree (as may be seen in Table C), when the two lots were grown out of doors and not subjected to any mutual competition. Never- theless, strange as is the fact, the flowers on the mother-plants of the third self-fertilised generation did not yield more seed when they were crossed with pollen from plants of the fresh stock than when they were self-fertilised. (11.) Anagallis coliina—Plants raised from a red variety crossed by another plant of the same variety were in height to the self-fertilised plants from the red variety as 100 to 73. When the flowers on the red variety were fertilised with pollen from a closely similar blue-flowered variety, they yielded double the number of seeds to what they did when crossed by pollen from another individual of the same red variety, and the seeds were much finer. The plants raised from this cross between the two varieties were to the self-fertilised seedlings from the red variety, in height as 100 to 66, and in fertility as 100 to 6. . (12.) Primula veris—Some flowers on long-styled plants of the third illegitimate generation were legiti- mately crossed with pollen from a fresh stock, and others were fertilised with their own pollen. From the seeds thus produced crossed plants, and self: 268 SUMMARY OF MEASUREMENTS. Caar. VIL. fertilised plants of the fourth illegitimate generation, were raised. The former were to the latter in height as 100 to 46, and in fertility during one year as 100 to 5, and as 100 to 3°5 during the next year. In this case, however, we have no means of distinguishing between the evil effects of illegitimate fertilisation continued during four generations (that is, by pollen of the same form, but taken from a distinct plant) and strict self-fertilisation. But these two processes perhaps do not differ so essentially as at first appears to be the case. In the following experi- ment any doubt arising from illegitimate fertilisation was completely eliminated. (18.) Primula veris. (Hqual-styled, red-flowered variety ).—F lowers on plants of the second self-fertilised generation were crossed with pollen from a distinct variety or fresh stock, and others were again self- fertilised. Crossed plants and plants of the third self- fertilised generation, all of legitimate origin, were thus raised ; and the former was to the latter in height as 100 to 85, and in fertility (as judged by the number of capsules produced, together with the average number of seeds) as 100 to 11. Summary of the Measurements in Table C—This table includes the heights and often the weights of 292 plants derived from a cross with a fresh stock, and of 305 plants, either of self-fertilised origin, or derived from an intercross between plants of the same stock. These 597 plants belong to thirteen species and twelve genera. The various precautions which were taken to ensure a fair comparison have already been stated. If we now look down the right-hand column, in which the mean height, weight, and fertility of the plants derived from a cross with a fresh stock are represented by 100, we shall sce Cuar. VIL. TABLE C. 269 by the other figures how wonderfully superior they are both to the self-fertilised and to the intercrossed plants of the same stock. With respect to height and weight, there are only two exceptions to the rule, namely, with Eschscholtzia and Petunia, and the latter is probably no real exception. Nor do these two species offer an exception in regard to fertility, for the plants derived from the cross with a fresh stock were much more fertile than the self-fertilised plants. The difference between the two sets of plants in the table is generally much greater in fertility than in height or weight. On the other hand, with some of the species, as with Nicotiana, there was no difference in fertility between the two sets, although a great dif- ference in height and weight. Considering all the cases in this table, there can be no doubt that plants profit immensely, though in different ways, by a cross with a fresh stock or with a distinct sub-variety. It cannot be maintained that the benefit thus derived is due merely to the plants of the fresh stock being per- fectly healthy, whilst those which had been long inter- crossed or self-fertilised had become unhealthy ; for in most cases there was no appearance of such unhealthi- ness, and we shall see under Table A that the inter- crossed plants of the same stock are generally superior to a certain extent to the self-fertilised,—both lots having been subjected to exactly the same conditions and being equally healthy or unhealthy. We further learn from Table C, that a cross between plants that have been self-fertilised during several successive generations and kept all the time under nearly uniform conditions, does not benefit the offspring in the least or only in a very slight degree. Mimulus and the descendants of Ipomcea named Hero offer instances of this rule. Again, plants self-fertilised 270 SUMMARY OF MEASUREMENTS. Cuar. VII. during several generations profit only to a small extent by a cross with intercrossed plants of the same stock (as in the case of Dianthus), in comparison with the effects of a cross by a fresh stock. Plants of the same stock intercrossed during several generations (as with Petunia) were inferior in a marked manner in fertility to those derived from the corresponding self-fertilised plants crossed by a fresh stock. Lastly, certain plants which are regularly intercrossed by insects in a state of nature, and which were artificially crossed in each succeeding generation in the course of my experi- ments, so that they can never or most rarely have suffered any evil from self-fertilisation (as with Esch- scholtzia and Ipomcea), nevertheless profited greatly by a cross with a fresh stock. These several cases taken together show us in the clearest manner that it is not the mere crossing of any two individuals which is beneficial to the offspring. The benefit thus derived depends on the plants which are united differing in some manner, and there can hardly be a doubt that it is in the constitution or nature of the sexual elements. Anyhow, it is certain that the differences are not of an external nature, for two plants which resemble each other as closely as the individuals of the same species ever do, profit in the plainest manner when inter- crossed, if their progenitors have been exposed during several generations to different conditions. But to this latter subject I shall have to recur in a future chapter. TABLE A. We will now turn to our first table, which re- lates to crossed and self-fertilised plants of the same stock. These consist of fifty-four species belonging to thirty natural orders. The total number of crossed plants of which measurements are given is 796, and Cuap. VII. TABLE A, 271 of self-fertilised plants 809; that is altogether 1,605 plants. Some of the species were experimented on during several successive generations ; and it should be borne in mind that in such cases the crossed plants in each generation were crossed with pollen from’ another crossed plant, and the flowers on the self- fertilised plants were almost always fertilised with their own pollen, though sometimes with pollen from other flowers on the same plant. The crossed plants thus became more or less closely inter-related in the later generations ; and both lots were subjected in each ge- neration to almost absolutely the same conditions, and to nearly the same conditions in the successive gene- rations. It would have been a better plan in some re spects if I had always crossed some flowers either on the self-fertilised or intercrossed plants of each generation with pollen from a non-related plant, grown under dif- ferent conditions, as was done with the plants in Table C; for by this procedure I should have learnt how much the offspring became deteriorated through continued self-fertilisation in the successive generations. As the case stands, the self-fertilised plants of the successive generations in Table A were put into competition with and compared with intercrossed plants, which were probably deteriorated in some degree by being more or less inter-related and grown under similar conditions. Nevertheless, had I always followed the plan in Table ©, I should not have discovered the important fact that, although a cross between plants which are rather closely related and which had been subjected to closely similar conditions, gives during several genera- tions some advantage to the offspring, yet that after a time they may be intercrossed with no advantage what- ever to the offspring. Nor should I have learnt that tlie self-fertilised plants of the later generations might 272 SUMMARY OF MEASUREMENTS. Cuar. VIL be crossed with intercrossed plants of the same stock with little or no advantage, although they profited to an extraordinary degree by a cross with a fresh stock. With respect to the greater number of the plants in Table A, nothing special need here be said; full particulars may be found under the head of each species by the aid of the Index. The figures in the right-hand column show the mean height of the self- fertilised plants, that of the crossed plants with which they competed being represented by 100. No notice is here taken of the few cases in which crossed and self-fertilised plants were grown in the open ground, so as not to compete together. The table includes, as we have seen, plants belonging to fifty-four species, but as some of these were measured during several successive generations, there are eighty-three cases in which crossed and self-fertilised plants were compared. As in each generation the number of plants which were measured (given in the table) was never very large and sometimes small, whenever in the right-hand column the mean height of the crossed and self-fertilised plants is the same within five per cent., their heights may be considered as practically equal. Of such cases, that is, of self-fertilised plants of which the mean height is expressed by figures between 95 and 105, there are eighteen, either in some one or all the gene- rations. There are eight cases in which the self- fertilised plants exceed the crossed by above five per cent., as shown by the figures in the right-hand column being above 105. Lastly, there are fifty-seven cases in which the crossed plants exceed the self-fertilised in a ratio of at least 100 to 95, and generally in a much higher degree. If the relative heights of the crossed and self-fertilised plants had been due to mere chance, there would haye Cuar. VIL. TABLE A. 273 been about as many cases of self-fertilised plants exceeding the crossed in height by above five per cent. as of the crossed thus exceeding the self-fertilised ; but we see that of the latter there are fifty-seven cases, and of the former only eight cases; so that the cases in which the crossed plants exceed in height the self- fertilised in the abeve proportion are more than seven times as numerous as those in which the self-fertilised exceed the crossed in the same proportion. For our special purpose of comparing the powers of growth of crossed and self-fertilised plants, it may be said that in fifty-seven cases the crossed plants exceeded the self-fertilised by more than five per cent., and that in twenty-six cases (18+8) they did not thus exceed them. But we shall now show that in several of these twenty-six cases the crossed plants had a decided ad- vantage over the self-fertilised in other respects, though not in height; that in other cases the mean heights are not trustworthy, owing to too few plants having been measured, or to their having grown unequally from being unhealthy, or to both causes combined. Nevertheless, as these cases are opposed to my general conclusion I have felt bound to give them. Lastly, the eause of the crossed plants having no advantage over the self-fertilised can be explained in some other cases. Thus a very small residue is left in which the self- fertilised plants appear, as far as my experiments serve, to be really equal or superior to the crossed plants. We will now consider in some little detail the eigh- teen cases in which the self-fertilised plants equalled in average height the crossed plants within five per cent.; and the eight cases in which the self-fertilised plants exceeded in average height the crossed plants by above five per cent.; making altogether twenty-six As 274 SUMMARY OF MEASUREMENTS. Cuap. VIL cases in which the crossed plants were not tailer than the self-fertilised plants in any marked degree. (1.) Dianthus caryophyllus (third generation).—This plant was experimented on during four generations, in three of which the crossed plants exceeded in height the self-fertilised generally by much more than five per cent.; and we have seen under Tablo C that the offspring from the plants of the third self-fertilised generation crossed by a fresh stock profited in height and fer- tility to an extraordinary degree. But in this third generation the crossed plants of the same stock were in height to the self- fertilised only as 100 to 99, that is, they were practically equal. Nevertheless, when the eight crossed and eight self-fertilised plants were cut down and weighed, the former were to the latter in weight as 100 to 49! There can therefore be not the least - doubt that the crossed plants of this species are greatly superior in vigour and luxuriance to the self-fertilised ; and what was the cause of the self-fertilised plants of the third generation, though so light and thin, growing up so as almost to equal the crossed in height, I cannot explain. (2.) Lobelia fulgens (first generation).—The crossed plants of this generation were much inferior in height to the selffertilised, in the proportion of 100 to 127. Although only two pairs were measured, which is obviously much too few to be trusted, yet from other evidence given under the head of this species, it is certain that the self-fertilised plants were very much more vigorous than the crossed. As I used pollen of unequal maturity for crossing and self-fertilising the parent-plants, it is possible that the great difference in the growth of their offspring may have been due to this cause. In the next generation this source of error was avoided, and many more plants were raised, and now the average height of the twenty-three crossed plants was to that of the twenty-three self-fertilised plants as 100 to 91. We can therefore hardly doubt that a cross is beneficial to this species. (3.) Petunia violacea (third generation).—Eight crossed plants were to eight self-fertilised of the third generation in average height as 100 to 131; and at an early age the crossed were inferior even in a still higher degree. But it is a remarkable fact that in one pot in which plants of both lots grew extremely crowded, the crossed were thrice as tall as the self-fertilised. As in the two preceding and two succeeding generations, as well as Cuar. VIL TABLE A. 275 with plants raised by a cross with a fresh stock, the crossed greatly exceeded the self-fertilised in height, weight, and fertility (when these two latter points were attended to), the present case must be looked at as an anomaly not affecting the gencral rule. The most probable explanation is that the plants grew pre- maturely, owing to the seeds of the last generation not having been well ripened; for I have observed an analogous case with Iberis. Self-fertilised seedlings of this latter plant, which were known to have been produced from seeds not well matured, grew from the first much more quickly than the crossed plants, which were raised from better matured seeds; so that having thus once got a great start they were enabled ever afterwards to retain their advantage. Some of these same seeds of the Iberis were sown on the opposite sides of pots filled with burnt earth and pure sand, not containing any organic matter; and now the young crossed seedlings grew during their short life to double the height of the self-fertilised,in the same manner as occurred with the above two sets of seedlings of Petunia which were much crowded and thus exposed to very unfavourable conditions. We have seen also in the eighth generation cf Ipomcea that self- fertilised seedlings raised from unhealthy parents grew at first very much more quickly than the crossed seedlings, so that they were for a long time much taller, though ultimately beaten by them. (4, 5, 6.) Lschscholtzia californica.—Four sets of measure- ments are given in Table A. In one of these the crossed plants exceed the self-fertilised in average height, so that this is not one of the exceptions here to be considered. In two other eases the crossed equalled the self-fertilised in height within five per cent.; andin the fourth case the self-fertilised exceeded the crossed by above this limit. Wehave scen in Table C that the whole advantage of a cross by a fresh stock is confined to the number of seeds produced, and so it was with the disadvantage from self-fertilisation with the intercrossed plants of the same. stock compared with the self-fertiliscd, for the former were in fertility to the latter as 100 to 89. The intercrossed plants thus have at least one important advantage over the scelf-fertilised. Moreover, the flowers on the parent-plants when fertilised with pollen from another individual of the same stock yield far more seeds than when self-fertilised; the flowers in this latter case being often quite sterile. We may therefore conclude that a cross does some good, though it does not give to the crossed seedlirgs increased powers of growth, 7; 2 276 SUMMARY OF MEASUREMENTS. Cuar. VIL (7.) Viscuria oculata.—The average height of the fifteen inter- crossed plants to that of the fifteen self-fertilised plants was only as 100 to 97; but the former produced many more capsules than the latter, in the ratio of 100 to 77. Moreover, the flowers on the parent-plants which were crossed and self-fertilised, yielded seeds on one occasion in the proportion of 100 to 88, and on a second occasion in the proportion of 100 to 58. So that there can be no doubt about the beneficial effects of a cross, although the mean height of the crossed plants was only three per cent. above that of the self-fertilised plants. (8.) Specularia speculum.—Only the four tallest of the crossed and the four tallest of the self-fertilised plants, growing in four pots, were measured ; and the former were to the latter in height as 100 to 98. In all four pots a crossed plant flowered before any one of the self-fertilised plants, and this is usually a safe indication of some real superiority in the crossed plants. The flowers on the pareut-plants which were crossed with pollen from another plant yielded seeds compared with the self-fertilised flowers in the ratio of 100 to 72. We may therefore draw the same conclusion as in the last case with respect to a cross being decidedly beneficial. (9.) Borago officinalis—Only four crossed and four self- fertilised plants were raised and measured, and the former were to the latter in height as 100 to 102. So small a number of measurements ought never to be trusted; and in the present in- stance the advantage of the self-fertilised over the crossed plants depended almost entirely on one of the self-fertilised plants haying grown to an unusual height. All four crossed plants flowered before their self-fertilised opponents. The cross- fertilised flowers on the parent-plants in comparison with the self-fertilised flowers yielded seeds in the proportion of 100 to 60. So that here again we may draw the same conclusion as in the two last cases. (10.) Passiflora gracilis—Only two crossed and two self- fertilised plants were raised; and the former were to the latter in height as 100 to 104. On the other hand, fruits from the cross- fertilised flowers on the parent-plants contained seeds in number, compared with those from the self-fertilised flowers, in the pro- portion of 100 to 85. (11.) Phaseolus multiflorus—The five crossed plants were to the five self-fertilised in height as 100 to 96. Although the crossed plants were thus only four per cent. taller than the Citar. VIL. TABLE A, pairs self-fertilised, they flowered in both pots before them. It is therefore probable that they had some real advantage over the self-fertilised plants. (12.) Adonis estivalis—The four crossed plants were almost exactly equal in height to the four self-fertilised plants, but as so few plants were measured, and as these were all “ miserably unhealthy,” nothing can be inferred with safety with respect to their relative heights. (18.) Bartonia aurea—The eight crossed plants were to the eight self-fertilised in height as 100 to 107. This number of plants, considering the care with which they were raised and compared, ought to have given a trustworthy result. But from some unknown cause they grew very unequally, and they be- came so unhealthy that only three of the crossed and three of the self-fertilised plants set any seeds, and these few in number. Under these circumstances the mean height of neither lot can be trusted, and the experiment is valueless. The cross- fertilised flowers on the parent-plants yielded rather more seeds than the self-fertilised flowers. (14.) Thunbergia alata.—The six crossed plants were to the six self-fertilised in height as100 to 108. Here the self-fertilised plants seem to have a decided advantage; but both lots grew unequally, some of the plants in both being more than twice as tall as others. The parent-plants also were in an odd semi- sterile condition, Under these circumstances the superiority of the self-fertilised plants cannot be fully trusted. (15.) Moana prostrata.—The five crossed plants were to the five self-fertilised in height as 100 to 105; so that the latter seem here to have a small but decided advantage. On the other hand, the flowers on the parent-plants which were cross-fertilised produced very many more capsules than the self-fertilised flowers, in the ratio of 100 to 21; and the seeds which the former con- tained were heavier than an equal number from the self-fertilised capsules in the ratio of 100 to 82. (16.) Hibiscus africanus——Only four pairs were raised, and the crossed were to the self-fertilised in height as 100 to 109. Excepting that too few plants were measured, I know of nothing else to cause distrust in the result. The cross-fertilised flowers on the parent-plants were, on the other hand, rather more productive than the self-fertilised flowers. 17.) Apium petroselinum.—A few plants (number not re- corded) derived from flowers believed to have been crossed by 278 SUMMARY OF MEASUREMENTS. — Cuar, VIL. insects and a few self-fertilised plants were grown on the opposite sides of four pots. They attained to a nearly equal height, the crossed having a very slight advantage. (18.) Vandellia nummular ifolia.—Twenty crossed! plants raised from the seeds of perfect flowers were to twenty self-fertilised plants, likewise raised from the seeds of perfect flowers, in height as 100 to 99. The experiment was repeated, with the sole difference that the plants were allowed to grow more crowded ; and now the twenty-four tallest of the crossed plants were to the twenty-four tallest self-fertilised plants in height as 100 to 94, and in weight as 100 to 97. Moreover, a larger number of the crossed than of the self-fertilised plants grew to a moderate height. ‘The above-mentioned twenty crossed plants were also grown in competition with twenty self-fertilised plants raised from the closed or cleistogamic flowers, and their heights were as 100 to 94. Therefore had it not been for the first trial, in which the crossed plants were to the self-fertilised in height only as 100 to 99, this species might have been classed with those in which the crossed plants exceed the self-fertilised by above five per cent. On the other hand, the crossed plants in the second trial bore fewer capsules, and these contained fewer seeds, than did the self-fertilised plants, all the capsules having been produced by cleistogamic flowers. The whole case therefore must be left doubtful. (19.) Pisem sativum (common pea).—Four plants derived from a cross between individuals of the same variety were in height to four self-fertilised plants belonging to the same varicty as 100 to 115. Although this cross did no good, we have seen under Table C that a cross between distinct varieties adds greatly to the height and vigour of the offspring ; and it was there explained that the fact of a cross between the individuals of the same variety not being beneficial, is almost certainly due to their having been self-fertilised for many generations, and in each generation grown under nearly similar conditions. (20, 21, 22.) Canna warscewiczi.—Plants belonging to three generations were observed, and in all of three the crossed were approximately equal to the self-fertilised ; the average height of the thirty-four crossed plants being to that of the same number of self-fertilised plants as 100 to 101. Therefore the crossed plants had no advantage over the self-fertilised ; and it is pro-. bable that the same explanation here holds good as in the case of Pisum sativum ; for the flowers of this Canna are perfectly Guar. VII. TABLE A. 279 self-fertile, and were never seen to be visited by insects in the hothouse, so as to be crossed by them. This plant, moreover, has been cultivated under glass for several generations in pots, and therefore under nearly uniform conditions. The capsules produced by the cross-fertilised flowers on the above thirty-four crossed plants contained more seeds than did the capsules produced by the self-fertilised flowers on the self-fertilised plants, in the proportion of 100 to 85; so that in this respect crossing was beneficial. (23.) Primula sinensis——The offspring of plants, some of which were legitimately and others illegitimately fertilised with pollen from a distinct plant, were almost exactly of the same height as the offspring of self-fertilised plants; but the former with rare exceptions flowered before the latter. I have shown in my work on heterostyled plants that this species is commonly raised in England from self-fertilised seed, and the plants from having been cultivated in pots have been subjected to nearly uniform conditions. Moreover, many of them are now varying and changing their character, so as to become in a greater or less degree equal-styled, and in consequence highly self-fertile. Therefore I believe that the cause of the crossed plants not exceeding in height the self-fertilised is the same as in the two previous cases of Piswm sativum and Canna. (24, 25, 26.) Nicotiana tabacum.—Four sets of measurements were made; in one, the self-fertilised plants greatly exceeded in height the crossed, in two others they were approximately equal to the crossed, and in the fourth were beaten by them; but this latter case does not here concern us. The individual plants differ in constitution, so that the descendants of some profit by their parents having been intercrossed, whilst others do not, Taking all three generations together, the twenty-seven crossed plants were in height to the twenty-seven self-fertilised plants as 100 to 96. This excess of height in the crossed plants is so small compared with that displayed by the offspring from the same mother-plants when crossed by a slightly different variety, that we may suspect (as explained under Table C) that most of the individuals belonging to the variety which served as the mother-plants in my experiments, had acquired a nearly similar constitution, so as not to profit by being mutually intercrossed. Reviewing these twenty-six cases, in which the crossed plants either do not exveea the self-fertilised 280 SUMMARY OF MEASUREMENTS. CuAr. VIL by above five per cent. in height, or are inferior to them, we may conclude that much the greater number of the cdses do not form real exceptions to the rule, —that a cross between two plants, unless these have been self-fertilised and exposed to nearly the same conditions for many generations, gives a_ great advantage of some kind to the offspring. Of the twenty-six cases, at least two, namely, those of Adonis and Bartonia, may be wholly excluded, as the trials were worthless from the extreme unhealthiness of the plants. In twelve other cases (three trials with Esch- scholtzia here included) the ercssed plants either were superior in height to the self-fertilised in all the other generations excepting the one in question, or they showed their superiority in some different manner, as in weight, fertility, or in flowering first; or again, the cross-fertilised flowers on the mother-plant were much more productive of seed than the self-fertilised. Deducting these fourteen cases, there remain twelve in which the crossed plants show no well-marked advantage over the self-fertilised. On the other hand, we have seen that there are fifty-seven cases in which the crossed plants exceed the self-fertilised in height by at least five per cent., and generally in a much higher degree. But even in the twelve cases just referred to, the want of any advantage on the crossed side is far from certain: with Thunbergia the parent- plants were in an odd semi-sterile condition, and the offspring grew very unequally; with Hibiscus and Apium much too few plants were raised for the measure- ments to pe trusted, and the cross-fertilised flowers of Hibiscus produced rather more seed than did the self- fertilised ; with Vandellia the crossed plants were a little taller and heavier than the self-fertilised, but as they were less fertile the case must be left doubtful. Cuap. VII. TABLE A, 281 Lastly, with Pisum, Primula, the three generations of Canna, and the three of Nicotiana (which together complete the twelve cases), a cross between two plants certainly did no good or very little good to the off- spring ; but we have reason to suspect that this is the result of these plants having been self-fertilised and cultivated under nearly uniform conditions for several] generations. The same result followed with the experi- mental plants of Ipomcea and Mimulus, and toa certain extent with some other species, which had been inten- tionally treated by me in this manner; yet we know that these species in their normal condition profit greatly by being intercrossed. There is, therefore, not a single case in Table A which affords decisive evidence against the rule that a cross between plants, the progenitors of which have been subjected to somewhat diversified conditions, is beneficial to the offspring. This is a surprising conclusion, for from the analogy of domesti- cated animals it could not have been anticipated, that the good effects of, crossing or the evil effects of self- fertilisation would have been perceptible until the plants had been thus treated for several generations. The results given in Table A may be looked at under another point of view. Hitherto each genera- tion has been considered as a separate case, of which there are eighty-three ; and this no doubt is the more correct method of comparing the crossed and self- fertilised plants. But in those cases in which plants of the same species were observed during several generations, a general average of their heights in all the generations together may be made; and such averages are given in Table A; for instance, under Ipomcea the general average for the plants of all ten generations is as 100 for the crossed, to 77 for the self-fertilised 282 SUMMARY OF MEASUREMENTS. Cuar. VII. plants. ‘This having been done in each case in which more than one generation was raised, it is easy to calculate the average of the average heights of the crossed and self-fertilised plants of all the species included in Table A. It should however be observed that as only a few plants of some species, whilst a considerable number of others, were measured, the value of the mean or average heights of the several species is very different. Subject to this source of error, it may be worth while to give the mean of the mean heights of the fifty-four species in Table A; and the result is, calling the mean of the mean heights of the crossed plants 100, that of the self-fertilised plants is 87. But it is a better plan to divide the fifty-four species into three groups, as was done with the previously given eighty-three cases. The first group consists of species of which the mean. heights of the self-fertilised plants are within five per cent. of 100; so that the crossed and self-fertilised plants are approximately equal; and of.such species there are twelve about which nothing need be said, the mean of the mean heights of the self-fertilised being of course very nearly 100, or exactly 99°58. The second group consists of the species, thirty-seven in number, of which the mean heights of the crossed plants exceed that of the self-fertilised plants by more than five per cent.; and the mean of their mean heights is to that of the self-fertilised plants as 100 to78. The third group consists of the species, only five in number, of which the mean heights of the self-fertilised plants exceed that of the crossed by more than five per cent.; and here the mean of the mean heights of the crossed plants is to that of the self-fertilised as 100 to109. Therefore if we exclude the species which are approximately equal, there are Cuar. VIL TABLE B. 283 thirty-seven species in which the mean of the mean heights of the crossed plants exceeds that of the self- fertilised by twenty-two per cent.; whereas there are only five species in which the mean of the mean heights of the self-fertilised plants exceeds that of the crossed, and this only by nine per cent. The truth of the conclusion—that the good effects of a cross depend on the plants having been subjected to different conditions or to their belonging to differ- ent varieties, in both of which cases they would aimost certainly differ somewhat in constitution—is supported by a comparison of the Tables A and C. The latter table gives the results of crossing plants with a fresh stock or with a distinct variety ; and the superiority of the crossed offspring over the self-fertilised is here much more general and much more strongly marked than in Table A, in which plants of the same stock were crossed. We have just seen that the mean of the mean heights of the crossed plants of the whole fifty- four species in Table A is to that of the self-fertilised plants as 100 to 87; whereas the mean of the mean heights of the plants crossed by a fresh stock is to that of the self-fertilised in Table C as 100 to 74. So that the crossed plants beat the self-fertilised plants by thirteen per cent. in Table A, and by twenty-six per cent., or double as much, in Table C, which includes the results of a cross by a fresh stock. TABLE B. A few words must be added on the weights of the crossed plants of the same stock, in comparison with the self-fertilised. Eleven cases are given in Table B, relating to eight species. The number of plants which were weighed is shown in the two left columns, and their relative weights in the right 284 SUMMARY OF MEASUREMENTS. Cuap. VIL column, that of the crossed plants being taken as 100. A few other cases have already been recorded in Table C in reference to plants crossed by a fresh stock. I regret that more trials of this kind were not made, as the evidence of the superiority of the crossed over the self-fertilised plants is thus shown in a more conclusive manner than by their relative heights. But this plan was not thought of until a rather late period, and there were difficulties in the way, as the seeds had to be collected when ripe, by which time the plants had often begun to wither. In only one out of the eleven cases in Table B, that of Eschscholtzia, do the self-fertilised plants exceed the crossed in weight; and we have already seen they are likewise superior to them in height, though inferior in fertility, the whole ad- vantage of a cross being here confined to the repro- ductive system. With Vandellia the crossed plants were a little heavier, as they were also a little taller than the self-fertilised; but as a greater number of more productive capsules, were produced by the cleis- togamic flowers on the self-fertilised plants than by those on the crossed plants, the case must be left, as remarked under Table A, altogether doubtful. The crossed and self-fertilised offspring from a partially self-sterile plant of Reseda odorata were almost equal in weight, though not in height. In the remaining eight cases, the crossed plants show a wonderful superiority over the self-fertilised, being more than double their weight, except in one case, and here the ratio is as high as 100 to 67. The results thus deduced from the weights of the plants confirm in a striking manner the former evidence of the beneficial effects of a cross between two plants of the same stock ; and in the few cases in which plants derived from a cross with a fresh stock were weighed, the results are similar or even more striking. Or Cuar. VIII. CONSTITUTIONAL VIGOUR. 28 CHAPTER VIL DIFFERENCE BETWEEN CrossED AND SELF-FERTILIEED PLANTS IN CONSTITUTIONAL VIGOUR AND IN OTHER RESPECTS, Greater constitutional vigour of crossed plants—The effects of great crowding—Competition with other kinds of plants—Scelf-fertilised plants more liable to premature death—Crossed plants generally flower before the self-fertilised Negative effects of intercrossing flowers on the same plant—Cases described—Transmission of the good effects of a cross to later generations—Hffects of crossing plants of closely related parentage—Uniform colour of the flowers on plants self-fertilised during several generations and cultivated under similar conditions. Greater constitutional Vigour of crossed Plants—As in almost all my experiments an equal number of crossed and self-fertilised seeds, or more commonly seedlings just beginning to sprout, were planted on the oppo- site sides of the same pots, they had to compete with one another ; and the greater height, weight, and fertility of the crossed plants may be attributed to their possessing greater innate constitutional vigour. Generally the plants of the two lots whilst very young were of equal height; but afterwards the crossed gained insensibly on their opponents, and this shows that they possessed some inherent superiority, though not displayed at a very early period of life. There were, however, some conspicuous exceptions to the rule of the two lots being at first equal in height; thus the crossed seedlings of the broom (Sarothamnus scoparius) when under three inches in height were more than twice as tall as the self-fertilised plants, Ye aa CONSTITUTIONAL VIGOUR Cuar. VIIL After the crossed or the self-fertilised plants had once grown decidedly taller than their opponents, a still increasing advantage would tend to follow from the stronger plants robbing the weaker ones of nourishment and overshadowing them. This was evidently the case with the crossed plants of Viola tri- color, which ultimately quite overwhelmed the self- fertilised. But that the crossed plants have an inherent superiority, independently of competition, was some- times well shown when both lots were planted separately, not far distant from one another, in good soil in the open ground. This was likewise shown in several cases, even with plants growing in close compe- tition with one another, by one of the self-fertilised plants exceeding for a time its crossed opponent, which had been injured by some accident or was at first sickly, but being ultimately conquered by it. The plants of the eighth generation of Ipomcea were raised from small seeds produced by unhealthy parents, and the self-fertilised plants grew at first very rapidly, so that when the plants of both lots were about three feet in height, the mean height of the crossed to that - of the self-fertilised was as 100 to 122; when they were about six fect high the two lots were very nearly equal, but ultimately when between eight and nine feet in height, the crossed plants asserted their usual superiority, and were to the self-fertilised in height as 100 to 89. The constitutional superiority of the crossed over the self-fertilised plants was proved in another way in the third generation of Mimulus, by self-fertilised seeds being sown on one side of a pot, and after a certain interval of time crossed seeds on the opposite side. ‘The self-fertilised seedlings thus had (for I ascertained that the seeds germinated simultaneously) a clear advantage Cuar. VIII. OF THE CROSSED PLANTS. 287 over the crossed in the start for the race. Nevertheless they were easily beaten (as may be seen under the head of Mimulus) when the crossed seeds were sown two whole days after the self-fertilised. But when the interval was four days, the two lots were nearly equal throughout life. Even in this latter case the crossed plants still possessed an inherent advantage, for after both lots had grown to their full height they were cut down, and without being disturbed were transferred to a larger pot, and when in the ensuing year they had again grown to their full height they were measured ; and now the tallest crossed plants were to the tallest self-fertilised plants in height as 100 to 75, and in fertility (i.e., by weight of seeds produced by an equal number of capsules from both lots) as 100 to 34. My usual method of proceeding, namely, to plant several pairs of crossed and self-fertilised seeds in an equal state of germination on the opposite sides of the same pots, so that the plants were subjected to moderately severe mutual competition, was I think the best that could have been followed, and was a fair test of what occurs in a state of nature. For plants sown by nature generally come up crowded, and are almost always exposed to very severe competition with one another and with other kinds of plants. This latter consideration led me to make some trials, chiefly but not exclusively with Ipomca and Mimulus, by sowing crossed and self-fertilised seeds on the opposite sides of large pots in which other plants had long been growing, or in the midst of other plants out of doors. The seedlings were thus subjected to very severe competition with plants of other kinds; and in all such cases, the crossed seedlings exhibited a great superiority in their power of growth over the selfs fertilised, 288 CONSTITUTIONAT, VIGOUR Cuar. VOL After the germinating seedlings had been planted in pairs on the opposite sides of several pots, the remaining seeds, whether or not in a state of germina- tion, were in most cases sown very thickly on the two sides of an additional large pot ; so that the seedlings came up extremely crowded, and were subjected to extremely severe competition and unfavourable condi- tions. In such cases the crossed plants almost invari- ably showed a greater superiority over the self-fertilised, than did the plants which grew in pairs in the pots. Sometimes crossed and self-fertilised seeds were sown in separate rows in the open ground, which was kept clear of weeds; so that the seedlings were not subjected to any competition with other kinds of plants. Those however in each row had to struggle with the adjoining ones in the same row. When fully grown, several of the tallest plants in each row were selected, measured, and compared. The result was in several cases (but not so invariably as might have been expected) that the crossed plants did not exceed in height the self-fertilised in nearly so great a degree as when grown in pairs in the pots. Jhus with the plants of Digitalis, which competed together in pots, the crossed were to the self-fertilised in height as 100 to 70; whilst those which were grown separately were only as 100 to 85. Nearly the same result was observed with Brassica. With Nicotiana the crossed were to the self-fertilised plants in height, when grown extremely crowded together in pots, as 100 to 54; when grown much less crowded in pots as 100 to 66, and when grown in the open ground, so as to be sub- jected to but little competition, as 100 to 72. On the other hand with Zea, there was a greater difference in height between the crossed and self-fertilised plants growing out of doors, than between the pairs which Cua. VIII. OF THE CROSSED PLANTS. 289 grew in pots in the hothouse; but this may be attributed to the self-fertilised plants being more tender, so that they suffered more than the crossed when both lots were exposed to a cold and wet summer, Lastly, with one out of two series of Reseda odorata, grown out of doors in rows, as well as with Beta vulgaris, the crossed plants did not at all exceed the self- fertilised in height, or exceeded them by a mere trifle. The innate power of the crossed plants to resist unfavourable conditions far better than did the self- fertilised plants, was shown on two occasions in a curious manner, namely, with Iberis and in the third generation of Petunia, by the great superiority in height of the crossed over the self-fertilised scedlings, when both sets were grown under extremely unfavourable conditions; whereas owing to special circumstances exactly the reverse occurred with the plants raised from the same seeds and grown in pairs in pots. A nearly analogous case was observed on two other occasions with plants of the first generation of Nicotiana. The crossed plants always withstood the injurious effects of being suddenly removed into the open air after having been kept in the greenhouse better than did the self-fertilised. On several occasions they also resisted much better cold and intemperate weather. This was manifestly the case with some crossed and self-fertilised plants of Ipomcea, which were suddenly moved from the hothouse to the coldest part of a cool greenhouse. The offspring of plants of the eighth self-fertilised generation of Mimulus crossed by a fresh stock, survived a frost which killed every single self- fertilised and intererossed plant of the same old stock. Nearly the same result followed with some crossed and self-fertilised plants of Viola tricolor. Even the tips of the shoots of the crossed plants «f Sarothamnus U _290 CONSTITUTIONAL VIGOUR Guar, VIIL scoparius were not touched by a very severe winter ; whereas all the self-fertilised plants were killed half- way down to the ground, so that they were not able to flower during the next summer. Young crossed seedlings of Nicotiana withstood a cold and wet summer much better than the self-fertilised seedlings. I have met with only one exception to the rule of crossed plants being hardier than the self-fertilised : three long rows of Eschscholtzia plants, consisting of crossed seedlings from a fresh stock, of intercrossed seedlings of the same stock, and of self-fertilised ones, were left unprotected during a severe winter, and all perished except two of the self-fertilised. But this case is not so anomalous as it at first appears, for it should be remembered that the self-fertilised plants of Eschscholtzia always grow taller and are heavier than the crossed; the whole benefit of a cross with this species being confined to increased fertility. Independently of any external cause which could be detected, the self-fertilised plants were more liable to premature death than were the crossed; and this seems to me a curious fact. Whilst the seedlings were very young, if one died its antagonist was pulled up and thrown away, and I believe that many more of the self-fertilised died at this early age than of the crossed ; but I neglected to keep any record. With Beta vulgaris, however, it is certain that a large number of the self-fertilised seeds perished after germinating beneath the ground, whereas the crossed seeds sown at the same time did not thus suffer. When a plant died at a somewhat more advanced age the fact was recorded ; and I find in my notes that out of several hundred plants, only seven of the crossed died, whilst of the self-fertilised at least twenty-nine were thus lost, that is more than four times as many. Mr. Galton, Cuar. VIL. OF THE CROSSED PLANTS. 291 after examining some of my tables, remarks: “ It is very evident that the columns with the self-fertilised plants include the larger number of exceptionally small plants ;” and the frequent presence of such puny plants no doubt stands in close relation with their lability to premature death. The self-fertilised plants of Petunia completed their growth and began to wither sooner than did the intercrossed plants; and these latter considerably before the offspring from a cross with a fresh stock. Period of Flowering.—In some cases, as with Digitalis, Dianthus, and Reseda, a larger number of the crossed than of the self-fertilised plants threw up flower-stems ; but this probably was merely the result of their greater power of growth; for in the first generation of Lobelia fulgens, in which the self-fertilised plants greatly ex- ceeded in height the crossed plants, some of the latter failed to throw up flower-stems. With a large number of species, the crossed plants exhibited a well-marked tendency to flower before the self-fertilised ones growing in the same pots. It should however be remarked that no record was kept of the flowering of many of the species; and when a record was kept, the flowering of the first plant in each pot was alone observed, although two or more pairs grew in the same pot. I will now give three lists,—one of the species in which the first plant that flowered was a crossed one,—a second in which the first that flowered was a self-fertilised plant,—and a third of those which flowered at the same time. Species, of which the first Plants that flowered were of Crossed Parentage. Ipomea purpurea.—t record in my notes that in all ten genera- tions many of the crossed plants flowered before the sclf- fertilised ; but no details were kept. G2 202 PERIOD OF FLOWERING OF Cuap. VIII. Mimulus luteus (First Generation).—Ten flowers on the crossed plants were fully expanded before one on the self-fertilised. Mimulus luteus (Second and Third Generation).—In both these generations a crossed plant flowered before one of the self- fertilised in all three pots. Mimulus luteus (Fifth Generation).—In all three pots a crossed plant flowered first; yet the self-fertilised plants, which belonged to the new tall variety, were in height to the crossed as 126 to 100. Mimulus luteus—Plants derived from a cross with a fresh stock, as well as the intercrossed plants of the old stock, flowered before the self-fertilised plants in nine out of the ten pots. Salvia coccinea.—A crossed plant flowered before any one of the self-fertilised in all three pots. Origanum vulgere—During two successive seasons several crossed plants flowered before the self-fertilised. Brassica oleracea (First Generation).—All the crossed plants growing in pots and in the open ground flowered first. Brassica oleracea (Second Generation).—A crossed plant in three out of the four pots flowered before any one of the self-fertilised. Iberis umbellata.—In both pots a crossed plant flowered first. Eschscholtzia californica.—Plants derived from the Brazilian stock crossed by the English stock flowered in five out of the nine pots first; in four of them a self-fertilised plant flowered first; and not in one pot did an intercrossed plant of the old stock flower first. Viola tricolor—A crossed plant in five out of the six pots flowered before any one of the self-fertilised. Dianthus caryophyllus (First Generation).—In two large beds of plants, four of the crossed plants flowered before any one of the self-fertilised. Dianthus caryophyllus (Second Generation)—In both pots a crossed plant flowered first. Dianthus caryophyllus (Third Generation).—In three out of the four pots a crossed plant flowered first; yet the crossed were to the self-fertilised in height only as 100 to 99, but in weight as 100 to 49. Dianthus caryophyllus.—Plants derived from a cross with a fresh stock, and the intercrossed plants of the old stock, both flowered before the self-fertilised in nine out of the ten pots. Hibiscus africanus.—In three out of the four pots a crossed Cuar. VIII. CROSSED AND SELF-FERTILISED PLANTS. 293 plant flowered before any one of the self-fertilised; yet the latter were to the crossed in height as 109 to 100. Tropxolum minus.—A crossed plant flowered before any one of the self-fertilised in three out of the four pots, and simul- taneously in the fourth pot. Limnanthes douglasii.—A crossed plant flowered before any one of the self-fertilised in four out of the five pots. Phaseolus multiflorus—In both pots a crossed plant flowered first. Specularia speculum.—In all four pots a crossed plant flowered first. Lobelia ramosa (First Generation).—In all four pots a crossed plant flowered before any one of the self-fertilised. Lobelia ramosa (Second Generation).—In all four pots a crossed plant flowered some days before any one of the self- fertilised. Nemophila insignis.—In four out of the five pots a crossed plant flowered first. Borago officinalis.—In both pots a crossed plant flowered first. Petunia violacea (Second Generation)—In all three pots a crossed plant flowered first. Nicotiana tabacum.—A plant derived from a cross with a fresh stock flowered before any one of the self-fertilised plants of the fourth generation, in fifteen out of the sixteen pots. Cyclamen persicum.—During two successive seasons a crossed plant flowered some weeks before any one of the self-fertilised in all four pots. Primula veris (equal-styled var.).—In all three pots a crossed plant flowered first. Primula sinensis—In all four pots plants derived from an illegitimate cross between distinct plants flowered before any one of the self-fertilised plants. Primula sinensis.—A. legitimately crossed plant flowered before any one of the self-fertilised plants in seven out of the eight pots. Fagopyrum esculentum.—A legitimately crossed plant flowered from one to two days before any one of the self-fertilised plants in all three pots. Zea mays.—In all four pots a crossed plant flowered first. Phalaris canariensis—The crossed plants flowered before the self-fertilised in the open ground, but simultancously in the pots. v94 PERIOD OF FLOWERING OF Cuar. VIII. Species, of which the first Plants that flowered were of Self-fertilised Parentage. Eschscholtzia californica (First Generation).—The crossed plants were at first taller than the self-fertilised, but on their second erowth during the following year the self-fertilised exceeded the crossed in height, and now they flowered first in three out of the four pots. Lupinus luteus.—Although the crossed plants were to the sclf- fertilised in height as 100 to 82; yet in all three pots the self-fertilised plants flowered first. Clarkia elegans—Although the crossed plants were, as in the last case, to the self-fertilised in height as 100 to 82, yet in the two pots the self-fertilised flowered first. Lobelia fulgens (First Generation).—The crossed plants were to the self-fertilised in height only as 100 to 127, and the latter flowered much before the crossed. Petunia violucea (Third -Generation).—The crossed plants were to the self-fertilised in height as 100 to 181, and in three out of the four pots a self-fertilised plant flowered first; in the fourth pot simultaneously. Petunia violacea (Fourth Generation).—Although the crossed plants were to the self-fertilised in height as 100 to 69,%et in three out of the five pots a self-fertilised plant flowered first; in the fourth pot simultaneously, and only in the fifth did acrossed plant flower first. Nicotiana tabacum (First Generatioh)—The crossed plants were to the self-fertilised in height only as 100 to 178, and a self-fertilised plant flowered first in all four pots. Nicotiana tabacum (Third Generation).—The crossed plants were to the self-fertilised in height as 100 to 101, and in four out of the five pots a self-fertilised plant flowered first. Canna warscewiczi—In the three generations taken together the crossed were to the self-fertilised in height as 100 to 101; in the first generation the self-fertilised plants showed some tendency to flower first, and in the third generation they flowered first in nine out of the twelve pots. Species in which the Crossed and Self-fertilised Plants flowered almost simultaneously. Aimulus luteus (Sixth Generation).—The crossed plants were inferior in height and vigour to the self-fertilised plants Cuar. VII. CROSSED AND SELF-FERTILISED PLANTS. 293 which all belonged to the new white-flowered tall variety, yet in only half the pots did the self-fertilised plants flower first, and in the other half the crossed plants, Viscarta oculata, —The crossed plants were only a little taller than the self-fertilised (viz., as 100 to 97), but con- siderably more fertile, yet both lots flowered almost simultaneously. Lathyrus odoratus (Second Generation).—Although the crossed plants were to the self-fertilised in height.as 100 to 88, yet there was no marked difference in their period of flowering. Lobelia fulgens (Second Generation).—Although the crossed plants were to the sclf-fertilised in height as 100 to 91, yet they flowered simultaneously. Nicotiana tabacum (Third Generation).—Although the crossed plants were to the self-fertilised in height as 100 to 83, yet in half the pots a self-fertilised plant flowered first, and in the other half a crossed plant. These three lists include fifty-eight cases, in which the period of flowering of the crossed and self-fertilised plants was recorded. In forty-four of them a crossed plant flowered first either in a majority of the pots or in all; in nine instances a self-fertilised plant flowered first, and in five the two lots flowered simultaneously. One of the most striking cases is that of Cyclamen, in which the crossed plants flowered some weeks before the self-fertilised in all four pots during two seasons. In the second generation of Lobelia ramosa, a crossed plant flowered in all four pots some days before any one of the self-fertilised. Plants derived from a cross with a fresh stock generally showed a very ~ strongly marked tendency to flower before the self- fertilised and the intercrossed plants of the old stock ; all three lots growing in the same pots. Thus with - Mimulis and Dianthus, in only one pot out of ten, and in Nicotiana in only one pot out of sixteen, did a self: fertilised plant flower before the plants of the two crossed kinds,—-these latter flowering almost simultaneously. 296 PERIOD OF FLOWERING OF Cuar. VIII. A consideration of the two first lists, especially of the second one, shows that a tendency to flower first is generally connected with greater power of growth, that is, with greater height. But there are some remarkable exceptions to this rule, proving that some other cause comes into play. Thus the crossed plants both of Lupinus luteus and Clarkia elegans were to the self- fertilised plants in height as 100 to 82, and yet the latter flowered first. In the third generation of Nico- tiana, and in all three generations of Canna, the crossed and self-fertilised plants were of nearly equal height, yet the self-fertilised tended to flower first. On the other hand, with Primula sinensis, plants raised from a cross between two distinct individuals, whether these were legitimately or illegitimately crossed, flowered before the illegitimately self-fertilised plants, although all the plants were of nearly equal height in both cases. So it was with respect to height and flowering with Phaseolus, Specularia, and Borago., The crossed plants of Hibiscus were inferior in height to the self-fertilised, in the ratio of 100 to 109, and yet they flowered before the self-fertilised in three out of the four pots. On the whole, there can be no doubt that the crossed plants exhibit a tendency to flower before the self-fertilised, almost though not quite so strongly marked as to grow to a greater height, to weigh more, and to be more fertile. A few other cases not included in the above three lists deserve notice. In all three pots of Viola tricolor, naturally crossed plants the offspring of crossed plants flowered before naturally crossed plants the offspring of self-fertilised plants. Flowers on two plants, both of self-fertilised parentage, of the sixth generation of Mimutlus luteus were intercrossed, and other flowers on the same plants were fertilised with their own pollen; Cuar. VIII. CROSSED AND SELF-FERTILISED PLANTS. 297 intercrossed seedlings and seedlings of the seventh self-fertilised generation were thus raised, and the latter flowered before the intercrossed in three out of the five pots. Flowers on a plant both of Mimulus luteus and of Ipomea purpurea were crossed with pollen from other flowers on the same plant, and other flowers were fertilised with their own pollen; intercrossed seedlings of this peculiar kind, and others strictly self- fertilised being thus raised. In the case of the Mimulus the self-fertilised plants flowered first in seven out of the eight pots, and in the case of the Ipomca in eight out of the ten pots; so that an intercross between the flowars on the same plant was very far from giving to the offspring thus raised, any advantage over the strictly self-fertilised plants in their period of flowering. The Kifects of crossing Flowers on the same Plant. In the discussion on the results of a cross with a fresh stock, given under Table C in the last chapter, it was shown that the mere act of crossing by itself does no good ; but that the advantages thus derived depend on the plants which are crossed, either consisting of distinct varieties which will almost certainly differ somewhat in constitution, or on the progenitors of the plants which are crossed, though identical in every external character, having been subjected to somewhat different conditions and having thus acquired some slight difference in constitution. All the flowers produced by the same plant have been developed from the same seed ; those which expand at the same time have been exposed to exactly the same climatic influences; and the stems have all been nourished by the same roots. ‘Therefore in accordance with the con- clusion just referred to, no good ought to result from 298 THE EFFECTS OF CROSSING Cuar. VII. crossing flowers on the same plant.* In oprosition to this conclusion is the fact that a bud is in one sense a distinct individual, and is capable of occasionally or even not rarely assuming new external characters, as well as new constitutional peculiarities. Plants raised from buds which have thus varied may be propagated for a great length of time by grafts, cuttings, &., and sometimes even by seminal generation.f There exist also numerous species in which the flowers on tho same plant differ from one another,—as in the sexual organs of moneecious and polygamous plants,—in the structure of the circumferential flowers in many Com- posite, Umbellifere, &¢.,.—in the structure of the central flower in some plants,—in the two kinds of flowers produced by cleistozamice species,—and in several other such cases. These instances clearly prove that the flowers on the same plant have often varied independently of one another in many im- portant respects, such variations having been fixed, * It is, however, possible that the stamens which differ in length or construction in the same flower may produce pollen differing in nature, and in this manner a cross might be made effective between the several flowers on the same plant. Mr. Macnab states (in a eommunication to M. Verlot, ‘ La Production des Variétés, 1865, p- 42) that seedlings raised from the shorter and longer stamens of rhododendron differ in character ; but the shorter stamens appa- rently are becoming rudimentary, and the seedlings are dwarfs, so that the result may be simply due to a want of fertilising power in the pollen, as in the case of the dwarfed plants of Mirabilis raised by Naudin by the use of too few pollen-grains, Analogous state- ments have been made with re- spect to the stamens of Pelargo- nium. With some of the Mela- stomaces, seedlings raised by me from flowers fertilised by pullen from the shorter stamens, cer- tainly differed in appearance from those raised from the longer sta- mens, with differently coloured anthers ; but here, again, there is some reason for believing that the shorter stamens are tending to- wards abortion. In the very dif- ferent case of trimorphice hetero- styled plants, the two sets of sta- mens in the same flower have widely different fertilising powers, t Ihave given numerous cases of such bud-variations in my ‘ Va- riation of Animals and Plants un- der Domestication,’ chap. xi, 2nd edit. vol. i. p. 448. Cuap. VIII. FLOWERS ON THE SAME PLANT. 299 like ‘those on distinct plants during the develop- ment of species. It was therefore necessary to ascertain by experiment what would be the effect of intercrossing flowers on the same plant, in comparison with fertilising them with their own pollen or crossing them with pollen from a distinct plant. Trials were carefully made on five genera belonging to four families; and in only one case, namely, Digitalis, did the offspring from a cross between the flowers on the same plant receive any benefit, and the benefit here was small compared with that derived from a cross between distinct plants. In the chapter on Fertility, when we consider the effects of cross-fertilisation and self-fertilisation on the productiveness of the parent-plants we shall arrive at nearly the same result, namely, that a cross between the flowers on the same plant does not at -all increase the number of the seeds, or only occasionally and to a slight degree. I will now give an abstract of the results of the five trials which were made. (1.) Digitalis purpurea.—Seedlings raised from inter- crossed flowers on the same plant, and others from flowers fertilised with their own pollen, were grown in the usual manner in competition with one another on the opposite sides of ten pots. In this and the four following cases, the details may be found under the head of each species. In eight pots, in which the plants did not grow much crowded, the flower-stems on sixteen intercrossed plants were in height to those on sixteen self-fertilised plants, as 100 to 94. In the two other pots in which the plants grew much crowded, the flower-stems on nine intercrossed plants were in height to those on nine self-fertilised plants, as 100 to 90. That the intercrossed plants in these two latter pots had a real advantage over their self-fertilised 300 THE EFFECTS OF CROSSING Cuar. VIII. opponents, was well shown by their relative weights when cut down, which was as 100 to 78. The mean height of the flower-stems on the twenty-five inter- crossed plants in the ten pots taken together, was to that of the flower-stems on the twenty-five self-ferti- lised plants, as 100 to 92. Thus the intercrossed plants were certainly superior to the self-fertilised in some degree; but their superiority was small compared with that of the offspring from a cross between distinct plants over the self-fertilised, this being in the ratio of 100 to 70 in height. Nor does this latter ratio show at all fairly the great superiority of the plants derived from a cross between distinct individuals over the self-fertilised, as the former produced more than twice as many flower-stems as the latter, and were much less liable to premature death. ; (2.) Ipomea purpurea.—Thirty-one intercrossed plants raised from a cross between flowers on the same plants were grown in ten pots in competition with the same number of self-fertilised plants, and the former were to the latter in height as 100 to 105. So that the seli-fertilised plants were a little taller than the inter- crossed ; and in eight out of the ten pots a self-fertilised plant flowered before any one of the crossed plants in the same pots. The plants which were not greatly crowded in nine of the pots (and these offer the fairest standard of comparison) were cut down and weighed ; and the weight of the twenty-seven intercrossed plants was to that of the twenty-seven self-fertilised as 100 to 124; so that by this test the superiority of the self-fertilised was strongly marked. To this subject of the superiority of the self-fertilised plants in certain cases, I shall have to recur in a future chapter. If we now turn to the offspring from a cross between distinct plants when put into competition with self- Caap. VILL. FLOWERS ON THE SAME PLANT. 801 fertilised plants, we find that the mean height of seventy-three such crossed plants, in the course of ten generations, was to that of the same number of self-fertilised plants as 100 to 77; and in the case of the plants of the tenth generation in weight as 100 to 44, Thus the contrast between the effects of crossing flowers on the same plant, and of crossing flowers on distinct plants, is wonderfully great. (3.) Mimulus luteus—Twenty-two plants raised by crossing flowers on the same plant were grown in competition with the same number of self-fertilised plants ; and the former were to the latter in height as 100 to 95, or if four dwarfed plants are excluded as 100 to101; and in weight as 100 to 103. In seven out of the eight pots a self-fertilised plant flowered before any of the intercrossed. So that here again the self- fertilised exhibit a trifling superiority over the inter- crossed plants. For the sake of comparison, I may add that seedlings raised during three generations from a cross between distinct plants were to the self- fertilised plants in height as 100 to 65. (4.) Pelargonium zonale—Two plants growing in separate pots, which had been propagated by cuttings from the same plant, and therefore formed in fact parts of the same individual, were intercrossed, and other flowers on one of these plants were self-fertilised ; but the seedlings obtained by the two processes did not differin height. When, on the other hand, flowers on one of the above plants were crossed with pollen taken from a distinct seedling, and other flowers were self-fertilised, the crossed offspring thus obtained were to the self-fertilised in height as 100 to 74. (5.) Origanum vulgare-—A plant which had been Jong cultivated in my kitchen garden, had spread by stolons so as to form a large bed or clump. Seedlings 302 THE EFFECTS OF CROSSING — Crap. VIII. raised by intercrossing flowers on these plants, which strictly consisted of the same plant, and other seedlings raised from self-fertilised flowers, were carefully com- pared from their earliest youth to maturity; and they did not differ at all in height or in constitutional vigour. Some flowers on these seedlings were then crossed with pollen taken from a distinct seedling, and other flowers were self-fertilised; two fresh lots of seedlings being thus raised, which were the grand- children of the plant that had spread by stolons and formed a large clump in my garden. These differed much in height, the crossed plants. being to the self-fertilised as 100 to 86. They differed, also, to a wonderful degree in constitutional vigour. The crossed plants flowered first, and produced exactly twice as many flower-stems; and they afterwards increased by stolons to such an extent as almost to overwhelm the self-fertilised plants. Reviewing these five cases, we see that in four of them, the effect of a cross between flowers on the same plant (even on offsets of the same plant growing on separate roots, as with the Pelargonium and Origa- num) does not differ from that of the strictest self- fertilisation. Indeed, in two of the cases the self-fer- tilised plants were slightly superior to such intercrossed plants. With Digitalis a cross between the flowers on the same plant certainly did do some good, yet very slight compared with that from a cross between distinct plants. On the whole the results here arrived at, if we bear in mind that the flower-buds are to a certain extent distinct individuals and occasionally vary inde- pendently of one another, agree well with our general conclusion, that the advantages of a cross depend on the progenitors of the crossed plants possessing some- what differént constitutions, either from haying been Cuar. VILL FLOWERS ON THE SAME PLANT. 303 exposed to different conditions, or to their having varied from unknown causes in a manner which we in our ignorance are forced to speak of as spontaneous. Hereafter I shall have to recur to this subject of the inefficiency of a cross between the flowers on the same plant, when we consider the part which insects play in the cross-fertilisation of flowers. On the Transmission of the good Effects from a Cross and of the evil Effects from Self-fertilisation—We have seen that seedlings from a cross between distinct plants almost always exceed their self-fertilised opponents in height, weight, and constitutional vigour, and, as_ will hereafter be shown, often in fertility. To. ascertain whether this superiority would be transmitted beyond the first generation, seedlings were raised on three occasions from crossed and self-fertilised plants, both sets being fertilised in the same manner, and therefore not as in the many cases given in Tables A, B, and ©, in which the crossed plants were again crossed and the self-fertilised again self-fertilised. Firstly, seedlings were raised from self-fertilised seeds produced under a net by crossed and self-fer- tilised plants of Nemophila insignis; and the latter were to the former in height as 133 to 100. But these seedlings became very unhealthy early in life, and grew so unequally that in both lots some were five times as tall as the others. Therefore this experiment was quite worthless; but I have felt bound to give it, as opposed to my general conclusion. I should state that in this and the two following trials, both sets of plants were grown on the opposite sides of the same pots, and treated in all respects alike. The details of the experi- ments may be found under the head of each species. Secondly, a crossed and a self-fertilised plant of Heartsease (Viola tricolor) grew near together in the 3804 TRANSMITTED EFFECTS OF A CROSS. Cuar. VIIL open ground and near to other plants of heartsease ; and as both produced an abundance of very fine cap- sules, the flowers on both were certainly cross-fertilised by insects. Seeds were collected from both plants, and seedlings raised from them. Those from the crossed plants flowered in all three pots before those from the self-fertilised plants; and when fully grown the former were to the latter in height as 100 to 82. As both sets of plants were the product of cross-fertilisation, the difference in their growth and period of flowering was clearly due to their parents having been of crossed and self-fertilised parentage; and it is equally clear that they transmitted different constitutional powers to their offspring, the grandchildren of the plants which were originally crossed and self-fertilised. Thirdly, the Sweet Pea (Lathyrus odoratus) habi- tually fertilises itself in this country. As I possessed plants, the parents and grandparents of which had been artificially crossed and other plants descended from the same parents which had been self-fertilised for many previous generations, these two lots of plants were allowed to fertilise themselves under a net, and their self-fertilised seeds saved. The seedlings thus raised were grown in competition with each other in the usual manner, and differed in their powers of growth. Those from the self-fertilised plants which had been crossed during the two previous generations were to those from the plants self-fertilised during many pre- vious generations in height as 100 to 90. These two lots of seeds were likewise tried by being sown under very unfavourable conditions in poor exhausted soil, and the plants whose grandparents and great-grandparents had been crossed showed in an unmistakable manner their superior constitutional vigour. In this case, as in that of the heartsease, there could be no doubt that Cuar. VIII. TRANSMITTED EFFECTS OF A CROSS. 305 the advantaze derived from a cross between two plants was not confined to the offspring of the first generation. That constitutional vigour due to cross-parentage is transmitted for many generations may also be inferred as highly probable, from some of Andrew Knight’s varieties of the common pea, which were raised by crossing distinct varieties, after which time they no doubt fertilised themselves in each succeeding gene- ration. These varieties lasted for upwards of sixty years, “ but their glory is now departed.”* On the other hand, most of the varieties of the common pea, which there is no reason to suppose owe their origin to a cross, have had a much shorter existence. Some also of Mr. Laxton’s varieties produced by artificial crosses have retained their astonishing vigour and luxuriance for a considerable number of generations ; but as Mr. Laxton informs me, his experience does not extend beyond twelve generations, within which period he has never perceived any diminuticn of vigour in his plants. An allied point may be here noticed. As» the force of inheritance is strong with plants (of which abundant evidence could be given), it is almost certain that seed- lings from the same capsule or from the same plant would tend to inherit nearly the same constitution ; and as the advantage from a cross depends on the plants which are crossed differing somewhat in constitution, it may be inferred as probable that under similar con- ditions a cross between the nearest relations would not benefit the offspring so much as one between non- related plants. In support of this conclusion we have some evidence, as Fritz Miller has shown by his * See the evidence on this mestication,’ chap. ix. vol. i. 2nd head in my ‘ Variation nnder Do- edit. p. 397, x 3806 UNIFORM COLOUR OF THE FLOWERS Cuwar. VIIL valuable experiments on hybrid Abutilons, that the union of brothers and sisters, parents and children, and of other near relations is highly injurious to the fertility of the offspring. In one case, moreover, seed- lings from such near relations possessed very weak constitutions.* ‘This same observer also foundf three plants of a Bignonia growing near together. He fer- tilised twenty-nine flowers on one of them with their own pollen, and they did not set a single capsule. Thirty flowers were then fertilised with pollen from a distinct plant, one of the three growing together, and they yielded onty two capsules. Lastly, five flowers were fertilised with pollen from a fourth plant growing at a distance, and all five produced capsules. It seems therefore probable, as Fritz Miller suggests, that the three plants growing near together were seedlings from the same parent, and that from being closely related they had little power of fertilising one another. Lastly, the fact of the intercrossed plants in Table A not exceeding in height the self-fertilised plants in a greater and greater degree in the later generations, 1s probably due to their having become more and more closely inter-related. Uniform Colour of the Flowers on Plants, self-fertilised and grown under similar conditions for several Genera- tions.—At the commencement of my experiments, the parent-plants of Memulus luteus, Ipomoea purpurea, Dianthus caryophyllus, and Petunia violacea, raised from purchased seeds, varied greatly in the colour * *Jenaische Zeitschrift fiir Domestication’ (chap. xvii. 2nd Naturw.; B. vii., pp.22 and 45, 1872; and 1873, pp. 441-450. + ‘Bot. Zeitung,’ 1868, p. 626. t Some remarkable case3 are given in my ‘Variation under edit. yol. 2, p. 121) of hybrids of Gladiolus and Cistus, any one of which could he fertilised by pollen from any other, but nct by its own pollen, Cuar. X. PLANTS STERILE WITHOUT INSECT-AID. 3863 on the summits of three of the spikes, which touched the net when the wind blew, produced a few seeds. This sterility was not due to the injurious effects of the net, for I fertilised five flowers with pollen from an adjoining plant, and these all yielded fine seeds. I removed the net, whilst one little branch still bore a few not completely faded flowers, and these were visited by bees and yielded seeds. S. coccinea.—Some covered-up plants produced a good many fruits, but not, I think, half as many as did the uncovered plants ; twenty-eight of the fruits spontaneously produced by the protected plant contained on an average only 1°45 seeds, whilst some artificially self-fertilised fruits on the same plant contained more than twice as many, viz.,3°3 seeds, Bignonia (unnamed species) (Bignoniacez).— — Quite sterile: see my account of self-sterile plants. Digitalis purpurea (Scrophulariaceze)— Extremely sterile, only a few poor capsules being produced. Linaria vulgaris (Scrophulariaceze).— Extremely sterile. Antirrhinum majus, red var. (Scrophulariacece).—Fifty pods gathered from a large plant under a net contained 9°8 grains weight of seeds; but many (unfortunately not counted) of the fifty pods contained no seeds. Fifty pods on a plant fully exposed to the visits of humble-bees contained 23-1 grains weight of seed, that is, more than twice the weight; but in this case again, several of the fifty pods contained no seeds. A. majus (white var., with a pink mouth to the corolla).— Fifty pods, of which only a very few were empty, on a covered-up plant contained 20 grains weight of seed; so that this variety seems to be much more self-fertile than the previous one. With Dr. W. Ogle (‘ Pop. Science Review, Jan. 1870, p. 52) a plant of this species was much more sterile when protected from insects than with me, for it produced only two small capsules. As showing the efficiency of bees, | may add that Mr. Crocker castrated some young flowers and left them uncovered; and these produced as many seeds as the unmutilated flowers. A majus (peloric var.).—This variety is quite fertile when arti- ficially fertilised with its own pollen, but is utterly sterile when left to itself and uncovered, as humble-bees cannot crawl intc the narrow tubular flowers. 364 PLANTS STERILE WITHOUT INSECT-AID. Cuar, X. Verbascum phoeniceum (Serophulariaceze).—Quite sterile) See my ac ° ° - count of self- V. nigrum.—Quite sterile 4 5 feount plants, Campanula carpathica (Lobeliacex).—Quite sterile. Lobelia ramosa (Lobeliaceze).— Quite sterile. L. fulgens.—This plant is never visited in my garden by bees, and is quite sterile; but in a nursery-garden at a few miles’ distance I saw humble-bees visiting the flowers, and they produced some capsules. Isotoma (a white-flowered var.) (Lobeliacez).—Five plants left unprotected in my greenhouse produced twenty-four fine capsules, containing altogether 12°2 grains weight of seed, and thirteen other very poor capsules, which were rejected. Five plants protected from insects, but otherwise exposed to the same conditions as the above plants, produced sixteen fine capsules, and twenty other very poor and rejected ones. ‘The sixteen fine capsules contained seeds by weight in such proportion that twenty-four would have yielded 4°66 grains. So that the unprotected plants produced nearly thrice as many seeds by weight as the protected plants. Leschenaultia formosa (Goodeniaces).—Quite sterile. My experi- ments on this plant, showing the necessity of insect aid, are given in the ‘ Gardeners’ Chronicle,’ 1871, p. 1166. Senecio cruentus (Composite).—Quite sterile: see my account of self-sterile plants. Hleterocentron mexicanum (Melastomacese).—Quite sterile; but this species and the following members of the group pro- duce plenty of seed when artificially self-fertilised. *hexia glandulosa (Melastomaceze).—Set spontaneously only twe or three capsules. Centradenia floribunda (Melastomaceze).—During some years pro- duced spontaneously two or three capsules, sometimes none. Pleroma (unnamed species from Kew) (Melastomacee).—During some years produced spontaneously two or three capsules, sometimes none. Monocheetum ensiferum (Melastomacez).—During some years pro- duced spontaneously two or three capsules, sometimes none. Hedychium (unnamed. species) (Marantacese).—Almost self-sterile without aid. Orchidece—An immense proportion of the species sterile, if insects are excluded. Cuar. X. PLANTS FERTILE WITHOUT INSECT-AID. — 365 List of Plants, which when protected from Insects are either quite Fertile, or yield more than half the Number of Seeds produced by unprotected Plants. Passiflora gracilis (Passifloraceze).—Produces many fruits, but these contain fewer seeds than fruits from intercrossed flowers. Brassica oleracea (Cruciferze).— Produces many capsules, but these generally not so rich in seed as those on uncovered plants. Raphanus sativus (Cruciferse).—Half of a large branching plant was covered by a net, and was as thickly covered with capsules as the other and unprotected half; but twenty of the capsules on the latter contained on an average 3°5 seeds, whilst twenty of the protected capsules contained only 1°85 seeds, that is, only a little more than half the number. This plant might perhaps have been more properly included in the former list. Iberis umbellata (Cruciferee).—Highly fertile. I. amara.—Highly fertile. Reseda odorata and lutea (Resedacese).—Certain individuals com- pletely self-fertile. Euryale ferox (Nymphzacez).—Professor Caspary informs me that this plant is highly self-fertile when insects are excluded. He remarks in the paper before referred to, that his plants (as well as those of the Victoria regia) produce only one flower at a time; and that as this species is an annual, and was introduced in 1809, it must have been self-fertilised for the last fifty-six generations; but Dr. Hooker assures me that to his knowledge it has been repeatedly introduced, and that at Kew the same plant both of the Euryale and of the Victoria produce several flowers at the same time. Nymphea (Nymphzacex).—Some species, as I am informed by Professor Caspary, are quite self-fertile when insects are excluded. Adonis estivalis (Ranunculacese).—Produces, according to Pro- fessor H. Hoffmann (‘Speciesfrage,’ p. 11), plenty of seeds when protected from insects. Ranunculus acris (Ranunculacese).—Produces plenty of seeds under a net. Paupaver somniferum (Papaveracese).—-Thirty capsules from un- covered plants yielded 15:5 grains weight of seed, and thirty capsules from covered-up plants, growing in the same bed, 366 PLANTS FERTILE WITHOUT INSECT-AID. Cuav. X. yielded 16°5 grains weight; so that the latter plants were more productive than the uncovered. Professor H. Hoffmann (‘Speciesfrage,’ 1875, p. 53) also found this species self-fertile when protected from insects. P. vagum.—Produced late in the summer plenty of seeds, which germinated well. P. a rgemonoides a Fy + | Accoraing to Hildebrand (‘ Jahr- Glaucium luteum (Papaveracen) “[ Semamen 2 ne Argemone ochroleuca (Papaveracer) .} are by no means sterile. Adlumia cirrhosa (Fumariaces).—Sets an abundance of capsules. Hypecoum procumbens (Fumariacese). Hildebrand says (idem), with respect to protected flowers, that “eine gute Frucht- biidung eintrete.” Fumaria officinalis (Fumariaces).—Covered-up and unprotected plants apparently produced an equal number of capsules, and the seeds of the former seemed to the eye equally good. I have often watched this plant, and so has Hildebrand, and we have never seen an insect visit the flowers. H. Miiller has likewise been struck with the rarity of the visits of insects to it, though he has sometimes seen hive-bees at work. The flowers may perhaps be visited by small moths, as is probably the case with the following species. F, capreolata.—Several large beds of this plant growing wild were watched by me during many days, but the flowers were never visited by any insects, though a humble-bee was once seen closely to inspect them. Nevertheless, as the nectary contains much nectar, especially in the evening, I felt convinced that they were visited, probably by moths. The petals do not naturally separate or open in the least; but they had been opened by some means in a certain pro- portion of the flowers, in the same manner as follows when a thick bristle is pushed into the nectary; so that in this respect they resemble the flowers of Corydalis lutea. Thirty- four heads, each including many flowers, were examined, and twenty of them had from one to four flowers, whilst fourteen had not a single flower thus opened. It is there- fore clear that some of the flowers had been visited by insects, while the majority had not; yet almost all produced capsules, Linum usiiatissimum (Linacez).—Appears to be quite fertile. H. Hoffmann, ‘ Bot. Zeitung,’ 1876, p. 565. Impatiens barbigera (Balsarninacee).—The flywers, though exe Cuar. X. ‘PLANTS FERTILE WITHOUT INSECT-AID. 367 cellently adapted for cross-fertilisation by the bees which freely visit them, set abundantly under a net. I. noli-me-tungere (Balsaminace).— This species produces cleis- togamic and perfect flowers. A plant was covered with a net, and some perfect flowers, marked with threads, produced eleven spontaneously self-fertilised capsules, which contained » on an average 8°45 seeds. I neglected to ascertain the number of seeds produced by perfect flowers exposed to the visits of insects, but I believe it is not greatly in excess of theaboveaverage. Mr. A. W. Bennett has carefully described the structure of the flowers of J. fwlva in ‘ Journal Linn. Soc.’ vol. xiii. Bot. 1872, p. 147. This latter species is said to be sterile with its own pollen (‘Gard. Chronicle, 1868, p. 1286), and if so, it presents a remarkable contrast with J. barbigerum and noli-me-tangere. Limnanthes douglasii (Geraniaceze).—Highly fertile. Viscaria oculata (Caryophyllacez).—Produces plenty of capsules with good seeds. Stellaria media (Caryophyllaceze).—Covered-up and uncovered plants produced an equal number of capsules, and the seeds in both appeared equally numerous and good. Beta vulgaris (Chenopodiaces).—Highly self-fertile. Vicia sativa (Leguminose).—Protected and unprotected plants produced an equal number of pods and equally fine seeds, If there was any difference between the two lots, the covered- up plants were the most productive. V. hirsuta. — This species bears the smallest flowers of any British leguminous plant. The result of covering up plants was exactly the same as in the last species. Pisum sativum (Leguminos).—Fully fertile. Lathyrus odoratus (Leguminose).—Fully fertile. L. nissolia—Fully fertile. Lupinus luteus (Leguminose).—Fairly productive. L. pilosus.—Produced plenty of pods. Ononis minutissima (Leguminose).—Twelve perfect flowers on a plant under a net were marked by threads, and produced eight pods, containing on an average 2°38 seeds. Pods produced by flowers visited by insects would probably have contained on an average 3°66 seeds, judging from the effects of artificial cross-fertilisation. Phaseolus vulgaris (Leguminose).—Quite fertile. Trifuium arvense (Leguminose).—The excessively small flowers 368 PLANTS FERTILE WITHOUT INSECT-AID. Cuar. X. are incessantly visited by hive and humble-bees. When insects were excluded the flower-heads seemed to produce as many and as fine seeds as the exposed heads. I. procumbens.—On one occasion covered-up plants seemed to yield as many seeds as the uncovered. On a second occasion sixty uncovered flower-heads yielded 9:1 grains weight of seeds, whilst sixty heads on protected plants yielded no jess than 17°7 grains; so that these latter plants were much more productive ; but this result I suppose was accidental. I have often watched this plant, and have never seen the flowers visited by insects; but I suspect that the flowers of this species, and more especially of Zrifolium minus, are frequented by small nocturnal moths which, as 1 hear from Mr. Bond, haunt the smaller clovers. Medicago lupulina (Leguminose).—On account of the danger of losing the seeds, I was forced to gather the pods before they were quite ripe; 150 flower-heads on plants visited by bees yielded pods weighiag 101 grains; whilst 150 heads on protected plants yielded pods weighing 77 grains. The inequality would probably have been greater if the mature seeds could have been all safely collected and compared. Ig. Urban (Keimung, Bluthen, &c., bei Medicago, 1873) has described the means of fertilisation in this genus, as has the Rev. G. Henslow in the ‘ Journal of Linn. Soc. Bot.’ vol. ix. 1866, pp. 827 and 355. Nicotiana tabacum (Solanacere).—Fully self-fertile. Ipomea purpurea (Convolvulacese).—Highly self-fertile. Leptosiphon androsaceus (Polemoniacese).—Plants under a net produced a good many capsules. Primula mollis (Primulacex).—A homomorphice species, self-fer- tile: J. Scott, in‘ Journal Linn. Soc. Bot.’ vol. viii. 1864, p. 120. Nolana prostrata (Nolanacese).—Plants covered up in the green- house, yielded seeds by weight compared with uncovered plants, the flowers of which were visited by many bees. in the ratio of 100 to 61. Ajuga reptans (Labiate).—Set a good many seeds; but none of the stems under a net produced so many as several un- covered stems growing closely by. Fuphrasia oficinalis (Scrophulariacez).—Covered-up plants pro= duced plenty of seed ; whether less than the exposed plants I cannot say. I saw two small Dipterous insects (Dolichepos nigripennis and Empis chioptera) repeatedly sucking the Cuar.X. PLANTS FERTILE WITHOUT INSECT-AID. 369 flowers ; as they crawled into them, they rubbed against the bristles which project from the anthers, and became dusted with pollen. Veronica agrestis (Scrophulariacex).—Covered-up plants produced an abundance of seeds. I do not know whether any insects visit the flowers; but I have observed Syrphidz repeatedly covered with pollen visiting the flowers of V. hederefolia and chameedrys. Mimulus luteus (Scrophulariacex).—Highly self-fertile. Culceolaria (greenhouse variety) (Scrophulariacese).—Highly self- fertile. Verbascum thapsus (Scrophulariacese).—Highly self-fertile. V. lychnitis—Highly self-fertile. Vandellia nummularifolia (Scrophulariaces).—Perfect flowers produce a good many capsules. Bartsia odontites (Scrophulariaceze).—Covered- up plants produced a good many seeds; but several of these were shrivelled, nor were they so numerous as those produced by unprotected plants, which were incessantly visited by hive and humble- bees. Specularia speculum (Lobeliacese).— Covered plants produced almost as many capsules as the uncovered. Lactuca sativa (Composite).—Covered plants produced some seeds, but the summer was wet and unfavourable. Galium aparine (Rubiacee).—Covered plants produced quite as many seeds as the uncovered. Apium petroselinum (Umbelliferze)—Covered plants apparently were as productive as the uncovered. Zea mays (Graminee).—A single plant in the greenhouse produced a good many grains. Canna warscewiczi (Marantacese).—Highly self-fertile. Orchidacee. — In Europe Ophrys apifera is as regularly self- fertilised as is any cleistogamic flower. In the United States, South Africa, and Australia there are a few species which are perfectly self-fertile. These several cases are given in the 2nd edit. of my work on the Fertilisation of Orchids. Allium cepa (blood red var.) (Liliaceze).—Four flower-heads were covered. with a net, and they produced somewhat fewer and smaller capsules thar those on the uncovered heads. The capsules were counted on one uncovered head, and were 289 in number; whilst those on a fine head from under the net were only 199. 28 370 MEANS OF CROSS-FERTILISATION. Cnap. X. Each of these lists contains by a mere accident the same number of genera, viz., forty-nine.* The genera in the first list include sixty-five species, and those in the second sixty species; the Orchidezw in both being excluded. If the genera in this latter order, as well as in the Asclepiadz and Apocynacez, had been included, the number of species which are sterile if insects are excluded would have been greatly increased; but the lists are confined to species which were actually expe- rimented on. ‘The results can be considered as only approximately accurate, for fertility is so variable a character, that each species ought to have been tried many times. The above number of species, namely, 125, is as nothing to the host of living plants; but the mere fact of more than half of them being sterile within the specified degree, when insects are excluded, is a striking one; for whenever pollen has to be carried from the anthers to the stigma in order to ensure full fertility, there is at least a good chance of cross-fertili- sation. I do not, however, believe that if all known plants were tried in the same manner, half would be found to be sterile within the specified limits; for * The plants in these two lists are entomophilous, or adapted for fertilisation by insects, with the exception of Zeaand Beta, which are anemophilous or fertilised by the wind. I may therefore here repeat that, according to Rim- pan (‘Landwirth. Jarbuch,’ B. vi. 1877, pp. 192-233, and p. 1073), Rye is sterile if the access of pollen from other plants is pre- vented ; whereas wheat and barley are quite fertile under these condi- tions. Rimpan states (p. 199) that the different varieties of wheat be- have differently with respect to self and cross-fertilisation. He removed at an early age all the anthers from the florets of one variety of wheat, which neverthe- less produced a considerable num- ber of grains, being fertilised by the surrounding plants. I state this fact, because Mr. A. S. Wil- son concludes from his excellent experiments (‘ Gardeners’ Chroni- cle’ 1874, March 21, p. 375) that wheat is invariably self-fertilised, and no doubt it is so generally. Mr. Wilson believes that all the pollen shed by the exserted anthers is absolutely useless. This is a conclusion which it would require very rigid proof to make me to admit. Car. X MEANS OF CROSS-FERTILISATION. OFL many flowers were selected for experiment which pre- sented some remarkable structure; and such flowers often require insect-aid. Thus out of the forty-nine genera in the first list, about thirty-two have flowers which are asymmetrical or present some remarkable peculiarity ; whilst in the second list, including species which are fully or moderately fertile when insects were excluded, only about twenty-one out of the forty-nine are asymmetrical or present any remarkable peculiarity. Means of cross-fertilisation— The most important of all the means by which pollen is carried from the anthers to the stigma of the same flower, or from flower to flower, are insects, belonging to the orders of Hymenoptera, Lepidoptera, and Diptera; and in some parts of the world, birds.* Next in importance, but * T will here give all the cases z known to me of birds fertilising flowers. In South Brazil, hum- ming - birds certainly fertilise various plants which are sterile without their aid: (Fritz Miiller, ‘ Bot. Zeit.’ 1870, pp. 274-5, and ‘Jen. Zeit. f. Naturwiss.’ B. vii. 1872, 24.) Long-beaked hum- ming-birds visit tle flowers of Brugmansia, whilst some of the short-beaked species often pene- trate its large corolla in order to obtain the nectar in an illegitimate manner, in the same manner as do bees in all parts of the world. It appears, indeed, that the beaks of humming-birds are specially adapted to the various kinds of flowers which they visit: on the Cordillera they suck the Salviz, and lacerate the flowers of the Tacsonie; in Nicaragua, Mr. Belt saw them sucking the flowers of Marcgravia and Ery- thrina, and thus they carried pollen from flower to flower. In North America they are said to frequent the flowers of Impatiens: (Gould, ‘ Introduction to the Tro- chilidie, 1861, pp. 15, 120; ‘ Gard. Chronicle,’ 1869, p. 389; ‘The Naturalist in Nicaragua,’ p. 129; ‘Journal of Linn. Soc. Bot.’ vol. xiii. 1872, p. 151.) I may add that I often saw in Chile a Mimus with its head yellow with pollen from, as I believe, a Cassia. I have been assured that at the Cape of Good Hope, Strelitzia is fertilised by the Nectarinide. There can hardly be a doubt that many Australian flowers are fer- tilised by the many honey-sucking birds of that country. Mr. Wal- lace remarks (Address to the Bio- logical Section, Brit. Assoc. 1876) that he has “often observed the beaks and faces of the brush- tongued lories of the Moluccas covered with pollen.” In New Zealand many specimens of the Anthornis melanura bad their heads coloured with pollen from the flowers of an endemic species of Fuchsia: (Potts, ‘ Transact. New Zealand Institute,’ vol. iii. 1870, p. 72.) 2 Wig 372 MLEANS OF CROSS-FERTILISATION. Cuapr. X, in a quite subordinate degree, is the wind; and with some aquatic plants, according to Delpino,* currents of water. The simple fact of the necessity in many cases of extraneous aid for the transport of the pollen, and the many contrivances for this purpose, render it highly probable that some great benefit is thus gained; and this conclusion has now been firmly established by the proved superiority in growth, vigour, and fertility of plants of crossed parentage over those of self-fertilised parentage. But we should always keep in mind that two somewhat opposed ends have to be gained; the first and more important one being the production of seeds by any means, and the second, cross-ferti- lisation. The advantages derived from cross-fertilisation throw a flood of light on most of the chief characters of flowers. We can thus understand their large size and bright colours, and in some cases the bright tints of the adjoining parts, such as the peduncles, bractew, even true leaves, as with Poinsettia, &e. By this means they are rendered conspicuous to insects, on the same principle that almost every fruit which is devoured by birds presents a strong contrast in colour with the green foliage, in order that it may be seen and its seeds freely disseminated. With some flowers con- spicuousness is gained at the expense even of the reproductive organs, as with the ray-florets of many Composit, the exterior flowers of Hydrangea, and the terminal flowers of the Feather-hyacinth or Muscari. There is also reason to believe, and this was the opinion of Sprengel, that flowers différ in colour in accordance with the kinds of insects which frequent them. * See also Dr. Ascherson’s interesting essay in ‘Bct. Zeitung,’ 1871, p. 444. Cuar.X. MEANS OF CROSS-FERTILISATION. 373 Not only do the bright colours of flowers serve to attract insects, but dark-coloured streaks and marks are often present, which Sprengel long ago maintained served as guides to the nectary. These marks follow the veins in the petals, or lie between them. ‘They may occur on only one, or on all excepting one or more of the upper or lower petals; or they may form a dark ring round the tubular part of the corolla, or be confined to the lips of an irregular flower. In the white varieties of many flowers, such as of Digitalis purpurea, Antirrhinum majus, several species of Dian- thus, Phlox, Myosotis, Rhododendron, Pelargonium, Primula, and Petunia, the marks generally persist, whilst the rest of the corolla has become of a pure white; but this may be due merely to their colour being more intense and thus less readily obliterated. Sprengel’s notion of the use of these marks as guides appeared to me for a long time fanciful; for insects, without such aid, readily discover the nectary and bite holes through it from the outside. They also discover the minute nectar-secreting glands on the stipules and leaves of certain plants. Moreover, some few plants, such as certain poppies, which are not nectariferous, have guiding marks; but we might perhaps expect that some few plants would retain traces of a former nectariferous condition. On the other hand, these marks are much more common on asymme- trical flowers, the entrance into which would be apt to puzzle insects, than on regular flowers. Sir J. Lubbock has also proved that bees readily distinguish colours, and that they lose much time if the position of honey which they have once visited be in the least changed.* The following case affords, I think, the best evidence * ‘ British W'ld Fc wers ‘n relation to Insects,’ 1875, p. 44. ol4 MEANS OF CROSS-FERTILISATION. Cuar. X. that these marks have really been developed in corre- lation with the nectary. The two upper petals of the common Pelargonium are thus marked near their bases; and I have repeatedly observed that when the flowers vary so as to become peloric or regular, they lose their nectaries and at the same time the dark marks. Wnen the nectary is only partially aborted, only one of the upper petals loses its mark. Therefore the nec- tary and these marks clearly stand in some sort of close relation to one another; and the simplest view is that they were developed together for a special purpose ; the only conceivable one being that the marks serve as a guide to the nectary. It is, however, evident from what has been already said, that insects could discover the nectar without the aid of guiding marks. They are of service to the plant, only by aiding insects to visit and suck a greater number of flowers within a given time than would otherwise be possible; and thus there will be a better chance of fertilisation by pollen brought from a distinct plant, and this we know is of paramount importance. The odours emitted by flowers attract insects, as I have observed in the case of plants covered by a muslin net. Nageli affixed artificial flowers to branches, scenting some with essential oils and leaving others unscented ; and insects were attracted to the former in an unmistakable manner.* It would appear that thev must be guided by the simultaneous action of sight and smell, for M. Plateaut found that excellently made, but not scented, artificial flowers never deceived them. It will be shown in the next chapter that the flowers of certain plants remain fully expanded for * ¢‘Hnstehung, &c., der Natur- Assoc. for the Advancement of hist. Art.’ 1865, p. 23. Science,’ 1876. t ‘Proceedings of the French Cuar. X. MEANS OF CROSS-FERTILISATION. ote days or weeks and do not attract any insects; and it is probable that they are neglected from not having as yet secreted any nectar or become odoriferous. Nature may be said occasionally to try on a large scale the same experiment as that by M. Plateau. Not a few flowers are both conspicuous and odoriferous. Of all colours, white is the prevailing one; and of white flowers a considerably larger proportion smell sweetly than of any other colour, namely, 14°6 per cent.; of red, only 8°2 per cent. are odoriferous.* The fact of a larger proportion of white flowers smelling sweetly may depend in part on those which are fertilised by moths requiring the double aid of conspicuousness in the dusk and of odour. Most flowers which are fer- tilised by crepuscular or nocturnal insects emit their odour chiefly or exclusively in the evening, and they are thus less likely to be visited and have their nectar stolen by ill-adapted diurnal insects. Some flowers, however, which are highly odoriferous depend solely on this quality for their fertilisation, such as the night- flowering stock (Hesperis) and some species of Daphne ; and these present the rare case of flowers which are fertilised by insects being obscurely coloured. The storage of a supply of nectar in a protected place - is manifestly connected with the visits of insects. So is the position which the stamens and pistils occupy, either permanently or at the proper period through their own movements ; for when mature they invariably stand in the pathway leading to the nectary. ‘The shape of the nectary and of the adjoining parts are likewise related to the particular kinds of insects which * The colours and odours of I have not seen their original the flowers of 4200 species have works, but a very full abstract been tabulated by Landgrabe, is given in Loudon’s ‘Gardeners’ and by Schiibler and Kohler. Mag.’ vol. xiii, 1837, p. 367. 376 MEANS OF CROSS-FERTILISATION. Cuap. X. habitually visit the flowers; this has been well shown by H. Miller by his comparison of lowland species which are chiefly visited by bees, with alpine species belonging to the same genera whick are visited by butterflies.* Flowers may also be adapted to certain kinds of insects, by secreting nectar particularly attrac- tive to them, and unattractive to other kinds; of which fact Epipactis latifolia offers the most striking instance known to me, as it is visited exclusively by wasps. Structures also exist, such as the hairs within the corolla of the foxglove (Digitalis), which apparently serve to exclude insects that are not well fitted to bring pollen from one flower to another.t I need say nothing here of the endless contrivances, such as the viscid glands attached to the pollen-masses of the Orchidez and Asclepiade, or the viscid or roughened state of the pollen-grains of many plants, or the irritability of their stamens which move when touched by insects, &c., —as all these contrivances evidently favour or ensure cross-fertilisation. All ordinary flowers are so far open that insects can force an entrance into them, notwithstanding that some, like the Snapdragon (Antirrhinum), various Papilionaceous and Fumariaceous flowers, are in appearance closed. It cannot be maintained that their openness is necessary for fertility, as cleistogamic flowers which are permanently closed yield a full complement of seeds. Pollen contains much nitrogen and phos- * ‘Nature,’ 1874, p. 110; 1875, p. 190; 1876, pp. 210, 289. &c.—proteci the flowers from the access of crawling or wingless + Belt, ‘The Naturalist in Ni- caragua,’ 1874, p. 132. Kerner has shown in his admirable essay, ‘Die Schutzmittel der Bliithen gegen unberufene Giste, 1826,’ that many structures—hairs, viscid glands, the position of the parts, insects, which would steal the nectar, and yet,as they do not commonly carry pollen from one plant to another, but only from flower to flower on the same plant, would confer no benefit te the species. n Cuar X. MEANS OF CROSS-FERTILISATION. 377 phoris—the two most precious of all the elements for the yrowth of plants—but in the case of most open flowers, a large quantity of pollen is consumed by pollen-devouring insects, and a large quantity is destroyed during long-continued rain. With many plants this latter evil is guarded against, as far as is possible, by the anthers opening only during dry weather,*—by the position and form of some or all of the petals,—by the presence of hairs, &c., and as Kerner has shown in his interesting essay, by the movements of the petals or of the whole flower during cold and wet weather. In order to compensate the loss of pollen in so many ways, the anthers produce a far larger amount than is necessary for the fertilisation of the same flower. I know this from my own experiments on Ipomea, given in the Introduction; and it is still more plainly shown by the astonishingly small quantity produced by cleistogamic flowers, which lose none of their pollen, in comparison with that produced by the open flowers borne by the same plants; and yet this small quantity - suffices for the fertilisation of all their numerous seeds. Mr. Hassall took pains in estimating the number of pollen-grains produced by a flower of the Dandelion (Leontodon), and found the number to be 248,600, and in a Peony 3,654,000.